The cartel recruiter slipped onto campus disguised as a janitor and then zeroed in on his target: a sophomore chemistry student.

The recruiter explained that the cartel was staffing up for a project, and that he’d heard good things about the young man.

“‘You’re good at what you do,’” the student recalled the recruiter saying. “‘You decide if you’re interested.’”

In their quest to build fentanyl empires, Mexican criminal groups are turning to an unusual talent pool: not hit men or corrupt police officers, but chemistry students studying at Mexican universities.

People who make fentanyl in cartel labs, who are called cooks, told The New York Times that they needed workers with advanced knowledge of chemistry to help make the drug stronger and “get more people hooked,” as one cook put it.

The cartels also have a more ambitious goal: to synthesize the chemical compounds, known as precursors, that are essential to making fentanyl, freeing them from having to import those raw materials from China.

If they succeed, U.S. officials say, it would mark a terrifying new phase in the fentanyl crisis, in which Mexican cartels have more control than ever over one of the deadliest drugs in recent history.

“It would make us the kings of Mexico,” said one chemistry student who has been cooking fentanyl for six months.

The Times interviewed seven fentanyl cooks, three chemistry students, two high-ranking operatives and a high-level recruiter. All of them work for the Sinaloa Cartel, which the U.S. government says is largely responsible for the fentanyl pouring over the southern border.

Those affiliated with the cartel put themselves in danger just by talking to The Times, and spoke on the condition of anonymity for fear of retaliation. Their accounts matched those of American Embassy officials who track cartel activities, including the role students are playing in cartel operations and how they are producing fentanyl. Times reporters spoke to a chemistry professor, who said the recruitment of his students was common.

The students said they had different jobs within the criminal group. Sometimes, they said, they run experiments to strengthen the drug or to create precursors. Other times, they say, they supervise or work alongside the cooks and assistants who produce fentanyl in bulk.

It’s unclear how widespread the recruitment of students has become, but the pursuit of trained chemists seems to have been influenced in part by the coronavirus pandemic.

A 2020 Mexican intelligence assessment, leaked by a hacker group, found that the Sinaloa Cartel appeared to be recruiting chemistry professors to develop fentanyl precursor chemicals after the pandemic slowed supply chains.

American law enforcement officials also said that many young chemists had been swept up in arrests at Mexican fentanyl labs in recent years. The arrested chemists told the authorities that they had been working on developing precursors and making the drug stronger, according to the officials.

A chemistry professor at a university in Sinaloa State said he knew that some students enrolled in chemistry classes just to become more familiar with skills needed to cook synthetic drugs. The professor, who requested anonymity for fear of reprisals, said he had identified students who fit that profile by their questions and reactions during his lectures.

“Sometimes when I am teaching them synthesis of pharmaceutical drugs, they openly ask me, ‘Hey, professor, when are you teaching us how to synthesize cocaine and other things?’” he said.

Eager to preserve cooperation on migration, the Biden administration avoided publicly urging Mexico to do more to dismantle the cartels. President-elect Donald J. Trump has promised a more aggressive approach, threatening to deploy the U.S. military to battle the criminals, and vowing last month to issue a 25 percent tariff on Mexican goods if the country doesn’t stop the flow of drugs and migrants across the border.

In response to the tariff threat, Mexico’s new president, Claudia Sheinbaum, said that “international collaboration” was needed to prevent the shipment of precursors to Mexico from “Asian countries.”

But as the cartels gain greater control of the fentanyl supply chain, U.S. officials say, it will become more difficult for law enforcement in both countries to stop the industrialized production of synthetic opioids in Mexico.

The cartels “know we are now focused on the illicit trafficking of these precursor chemicals around the world,” said Todd Robinson, the State Department’s assistant secretary of the Bureau of International Narcotics and Law Enforcement Affairs.

Those efforts are driving the cartels “to try to bring this thing in-house,” Mr. Robinson said. “The practical result of that is their ability to more easily and quickly transfer those drugs to the United States.”

Mass producing fentanyl can be relatively straightforward if cartels are just mixing up imported precursors, experts said, because it’s easy to find instructions for producing the drug using those chemicals.

But trying to synthesize the precursors from scratch is a much more difficult process that requires a broader array of chemical techniques and skills, said James DeFrancesco, a forensic science professor at Loyola University Chicago who worked as a forensic chemist at the U.S. Drug Enforcement Administration for 18 years.

The process is also dangerous. Cooks and students said that even though they wore gas masks and hazmat suits, the risks they face are many: toxic exposure to the lethal drug, accidental explosions, mistakes that enrage their armed and extremely violent bosses.

Yet the work pays more than many legal jobs in chemistry, and that’s often enough of a sell. The second-year student said the recruiter who visited the campus had offered him $800 up front, plus a monthly salary of $800 — twice as much as the average pay for chemists formally employed in Mexico, according to government data.

The 19-year-old, raised in one of the poorest parts of Sinaloa, said he had chosen to study chemistry because his father had cancer and he wanted to help find a cure.

“I want to help people, not kill them,” he said. The idea of making a product that would lead to mass death made him sick — and yet the treatment his father needed was impossible for the family to afford.

He told the recruiter he was interested, and five days later he was picked up by cartel members, blindfolded and driven to a clandestine lab hidden in the mountains, he said.

The Recruiter

Before the Sinaloa Cartel ever approaches a recruit, it scouts out its prospect.

The ideal candidate is someone who has both classroom knowledge and street smarts, a go-getter who won’t blanch at the idea of producing a lethal drug and, above all, someone discreet, said one recruiter in an interview.

In months of searching, he said, he’s found three students who now work for him developing precursors. Many young people just don’t meet his standards.

“Some are lazy, some aren’t bright, some talk too much,” said the recruiter, a lanky middle-aged man with square glasses, who has worked for the cartel for 10 years. He described himself as fix-it man, focused on improving quality and output in the fentanyl business.

To identify potential candidates, the cartel does a round of outreach with friends, acquaintances and colleagues, the recruiter said, then talks to the targets’ families, their friends, even people they play soccer with — all to learn whether they’d be open to doing this kind of work. If the recruiter finds someone particularly promising, he might offer to cover the student’s tuition cost.

“We are a company; what a company does is invest in their best people,” he said.

When the cartel began mass-producing fentanyl about a decade ago, the recruiter said, it relied on uneducated cooks from the countryside who could easily get their hands on what people in the business call “recipes” for making the drug.

Compared to methamphetamine, a drug that requires more advanced equipment and expertise to manufacture at scale, fentanyl is straightforward to produce if precursor chemicals are available.

“It takes four steps,” said one longtime cook, laying out the process with the simplicity that might be found on the back of a box of cake mix. “You shake it up, mix it, dry it, wash it with acetone.”

But things got more complicated in recent years. China moved to restrict the export of fentanyl precursors, Mexico cracked down on imports of the chemicals and the coronavirus pandemic gummed up supply chains so that those ingredients became harder to find.

The recruiter and all three students interviewed said they hadn’t successfully produced precursors yet.

“We are close, but it’s not easy,” said one former student, a 21-year-old who started working in a lab this year. Baby-faced and bright-eyed, the student had dropped out of school to work for the cartel. “We need to keep doing tests and more tests.”

But the recruiter said the students had been helpful in one key respect: making the fentanyl even more potent.

Student No. 1

About a year ago, a relative approached a first-year chemistry student with a proposal: Wouldn’t she love to make real money as a fentanyl cook?

In an interview, the student said her relative had worked for the Sinaloa Cartel for years and knew exactly what to say to lure the young woman, the eldest of five siblings. Her mother was raising the children alone, cleaning houses 12 hours a day.

The cartel offered the student $1,000 as a signing bonus, the woman said. She was terrified, but she said yes. The lab where she works is about an hour’s flight from Sinaloa’s capital, on the small aircraft the cartel uses to transport cooks to work. Her bosses told her that her job was to manufacture more powerful fentanyl, she said.

The fentanyl coming out of Mexico has often been of low purity, a problem the recruiter attributes to the desperate rush to satisfy Americans’ appetite for the synthetic opioid.

“There was such an explosion of demand that many people just wanted to earn money, and those manufacturers just made whatever without caring about quality,” the recruiter said. But in a competitive market, he said, the cartel can win over more clients with a stronger drug.

The first-year student said she had experimented with all manner of concoctions to increase fentanyl’s potency, including mixing it with animal anesthetics. But none of her attempts at producing fentanyl precursors have worked.

“You’re starting from a blank page,” she said. “How do we create something we didn’t invent?”

Student No. 2

When he first arrived at work, the sophomore chemistry student who had been recruited on campus had no idea what he was supposed to be doing. He said the lab was in the mountains, in the midst of trees and covered by a tarp that had been painted to look like foliage, so it couldn’t be seen from a helicopter.

After three days of work, he said, one of the men in charge told him that he wasn’t there to make fentanyl. He was the newest member of a research and development lab, where everyone was working to figure out how to make precursors from scratch. He said he immediately started worrying about inadvertently causing an explosion.

“They don’t tell you how to do it — they say, ‘These are the products, you’re going to make them with this, it could go wrong, but that’s why you’re studying,’” he said.

The sophomore works with six others, three students from his class in university, and three older men who are not trained chemists. The work is a lot riskier than what he does in school, when he has time to attend.

“Here, if they don’t like what you produce, they can make you disappear,” he said.

A cartel boss recently visited the lab to praise his work, the student said, telling him that if he was able to help produce precursors successfully, the group would give him a house or a car, whatever he wanted.

The sophomore told them what he needed most was money for his dad. He kept his day job a secret from his father.

“When he asks questions, I lie and say I’m working at a company,” the sophomore said. “I think if he knew, he wouldn’t accept the money.”

They stood on a concrete platform over a cobblestone plaza as slave traders cast their final judgment, gazing westward at a bend in the mighty Cuanza River, where unknown horrors lay ahead.

For the ancestors of millions of African Americans, this slave market in Massangano, a village in Angola, was likely the place where they were sold into bondage. It was a point of no return.

Historians believe that people from the southern African nation of Angola accounted for one of the largest numbers of enslaved Africans shipped to the United States, including the first to arrive at Point Comfort, Va., in 1619.

That history has largely gone unnoticed in Angola and the United States, where many Black Americans often make pilgrimages to Ghana and Senegal in West Africa to trace their ancestors’ treacherous journeys but not to Angola.

Angola is trying to change that.

The country’s ministry of tourism is developing a global campaign to highlight the significance of Massangano. The ministry is also partnering with the United Nations Development Program and the American Chamber of Commerce in Angola to launch a crowdfunding campaign to rehabilitate the village and its historical sites. Angola’s president, João Lourenço, has asked his government to repair the lone dirt road to Massangano that becomes impassable with heavy rain.

The government has applied for the Cuanza River corridor to be declared a UNESCO World Heritage site. “This is the place where African Americans came from,” said Márcio de Jesus Lopes Daniel, Angola’s tourism minister. “Come and see where your roots are.”

There are also hopes that this history will draw the United States and Angola closer together diplomatically.

When President Biden travels to Angola this week, he is scheduled to visit the National Slavery Museum near the capital, Luanda, to highlight the bond between the two nations that was born out of slavery. A vast majority of African Americans have Angolan ancestry, said Stephen Lubkemann, an anthropology professor at George Washington University. In the battle for influence in mineral- and oil-rich Angola, that gives the United States an ability to draw historical and cultural ties to the country in ways that China, its rival, cannot.

Mr. Biden’s delegation is expected to include Wanda Tucker, whom Angolan government officials glowingly refer to as one of their own. She traced her ancestors to the first ship that docked at Point Comfort and has visited the country several times.

When he visited Angola last year, Defense Secretary Lloyd J. Austin III, the first African American to hold the position, drew a blunt connection between himself and the Angolans he was addressing. “Four centuries ago, slavers from far away put the men and women and children of this country into shackles — people who looked just like you and me,” he said.

About a quarter of all enslaved Africans confirmed to have arrived in the United States came from a region that included Angola, according to SlaveVoyages, a digital database. That is more than anywhere else in Africa.

Because Massangano sits at the intersection of Angola’s largest river, the Cuanza, and a major tributary, it was the country’s main transit point for trafficking captives to the coast, scholars said. Today, Massangano is a quiet village of a couple of hundred residents.

Behind a wall made of mud blocks on the edge of the village is a large wooden deck covered by a thatched roof and surrounded by empty concrete bungalows — a restaurant and lodge that an Angolan businessman hopes to open by January.

Most residents live at the bottom of a hill in homes made of mud and logs. Some blast music at night, others gather beneath trees and play loto, a game similar to bingo.

At the top of the hill, European Renaissance-style stone buildings sit mostly in ruins. Each one features a hand-painted sign identifying its purpose: the old town hall, the fort of Massangano. The slave market, at the village’s highest point, is marked by a concrete cross standing about two stories tall, at the spot where enslaved people in shackles would have stood before being sold.

“They cry. Always cry,” Afonso Vita, a historian who works for Angola’s tourism ministry, said of African American visitors.

The effort to elevate Angola’s history in the slave trade has prompted new awareness and conversation nationally, local historians said. The legacy of the slave trade is rarely discussed in Angola, in part because its consequences are not as easily felt as in the United States, where many African Americans are aware of lingering racial disparities, said Vladimiro Fortuna, the director of the National Slavery Museum in Luanda.

Mr. Fortuna said that by next year he hopes to have a plan in place to construct a new, larger slavery museum. Visitors to Luanda are increasingly touring sites related to the slave trade. That includes the Street of Flowers, where slave traders once laid flowers to cover the blood of brutalized enslaved people.

Mr. Vita, the historian, said that when he gave lectures about the atrocities endured by the enslaved, Angolans became visibly angry. “The time is right,” he said, “for us to start a revolution to reclaim our history.”

Gilberto Neto contributed reporting from Massangano, and Zolan Kanno-Youngs from Washington.

Rebel forces advanced in Syria on Sunday amid fierce fighting, capturing the airport of the major city of Aleppo and attacking the outskirts of the western city of Hama, according to local officials and the Syrian Observatory for Human Rights. Government troops loyal to President Bashar al-Assad were trying to repel them, they said.

The rebels had captured much of Aleppo a day earlier in a surprise offensive. They now control a broad swath of land across the provinces of Hama, Idlib and Aleppo, in the west and northwest of Syria, according to information from local officials and the Observatory, a Britain-based war monitor.

The New York Times also observed rebels in control of parts of Hama Province as well as neighborhoods in the east of the city of Aleppo and parts of the countryside beyond it that government forces had held only days earlier.

Government troops were battling to defend the city of Hama from being overrun, according to the Observatory. Syrian government warplanes were also bombing territory that was now held by the rebels, causing civilian casualties, the monitor said.

It said that government forces were receiving support from Russian fighter jets, which were striking targets across the countryside near Hama and Idlib province.

Russia, which is allied with Mr. Assad, has repeatedly come to his aid since early in the civil war that broke out in 2011, after protests over Mr. Assad’s autocratic rule drew a swift and bloody military crackdown. Mr. al-Assad has also counted on military and political support from Iran.

The rebel alliance is led by the group Hayat Tahrir al-Sham, which was once linked with Al Qaeda but publicly broke with the terrorist group years ago. Turkish-backed rebel groups have also joined in.

The Biden administration said on Saturday that it was closely monitoring the situation in Syria. “The United States has nothing to do with this offensive,” a National Security Council spokesman, Sean Savett, said in a statement, calling Hayat Tahrir al-Sham “a designated terrorist organization.”

“The United States, together with its partners and allies, urge de-escalation, protection of civilians and minority groups, and a serious and credible political process that can end this civil war once and for all,” Mr. Savett said.

A Syrian government statement said that Mr. Assad had spoken to the leaders of the United Arab Emirates and Iraq on Saturday, vowing that Syria would “defeat the terrorists, regardless of the intensity of their attacks.” Syrian officials routinely refer to rebels as terrorists.

Iran’s president, Masoud Pezeshkian, referred to the situation in Syria in Parliament on Sunday, saying that “Islamic countries must intervene to prevent America and Israel from exploiting the internal conflicts of countries and prevent the continuation of these crises.”

And the Iranian foreign minister, Abbas Araghchi, said he was heading to Damascus on Sunday. Ali Moujani, an Iranian diplomat, said on X that the foreign minister was making the trip in a show of support for the Syrian government.

The Syrian military said in a statement on Saturday that its operation to push back the rebels was “successfully” progressing and that it would soon initiate a counterattack. It tried to discredit reports about rebel advances, saying that the armed groups were spreading “false news” to “undermine the morale of our people and our brave army.”

Across the territory that had flipped back to the rebels, people could be seen tearing up Syrian government flags and pictures of Mr. Assad, including fighters and former Aleppo residents who were returning to their homes for the first time in years. Photos taken in Aleppo also showed the toppling of a statue that had apparently depicted Bassel al-Assad, the president’s elder brother.

Hwaida Saad contributed reporting from Beirut, Lebanon, Rania Khaled from Cairo and Leily Nikounazar from Brussels.

A former Israeli defense minister has accused Israel of committing war crimes and ethnic cleansing in the Gaza Strip, a rare critique from a member of the security establishment at a time of war.

The comments by Moshe Yaalon came amid mounting criticism of the Israeli military’s conduct in Gaza. They were swiftly denied and condemned by allies of Prime Minister Benjamin Netanyahu of Israel, saying that they would hurt the country and help its enemies.

Mr. Yaalon served as the Israeli military’s chief of staff during the second intifada and as Mr. Netanyahu’s defense minister during the 2014 war in Gaza, the longest conflict between Israel and Hamas before the current war. But he broke with Mr. Netanyahu in 2016 and has since become a critic of the Israeli leader.

At an event on Saturday, Mr. Yaalon denounced Mr. Netanyahu’s government for its actions in Gaza.

“The path they’re dragging us down is to occupy, annex, and ethnically cleanse — look at the northern strip,” he said. He also said Israel was being pulled in the direction of building settlements in Gaza, a notion that is supported by far-right politicians in Mr. Netanyahu’s government.

When the interviewer at the event asked Mr. Yaalon to clarify whether he thought Israel was on the way to carrying out ethnic cleansing, he responded: “Why on the way? What’s happening there? What’s happening there?”

“There’s no Beit Lahia. There’s no Beit Hanoun. They’re now operating in Jabaliya. They’re basically cleaning the territory of Arabs,” he said, referring to towns and cities in northern Gaza where a renewed Israeli offensive against the militant group Hamas has caused extensive damage in recent months. Tens of thousands of Palestinians have been killed in Gaza since the war began in response to the deadly Hamas-led attack on Israel in October 2023.

Mr. Yaalon doubled down on his accusations on Sunday, saying on public radio that Mr. Netanyahu’s government was exposing Israeli commanders to lawsuits at the International Criminal Court and was putting their lives at risk.

“I’m speaking in the name of IDF commanders who are operating in the northern strip,” Mr. Yaalon told the Reshet Bet radio station. “They reached out to me expressing fear about what’s happening there.”

He later said, in an apparent reference to the government: “At the end of the day, they’re perpetrating war crimes” — while making clear that his issue was not with the soldiers themselves.

The Israeli military declined to comment on Mr. Yaalon’s accusations, which came ten days after the International Criminal Court issued arrest warrants for Mr. Netanyahu and Yoav Gallant, his former defense minister for crimes against humanity and war crimes in Gaza.

Mr. Netanyahu’s office has rejected the accusations against the men in the warrants, calling them “absurd and false” and accusing the court of being motivated by antisemitism.

Mr. Yaalon’s comments were condemned by Mr. Netanyahu’s Likud party, of which Mr. Yaalon is a former member.

“Yaalon already lost his way a long time ago,” said the party. “His defamatory words are a prize for the International Criminal Court and the haters of Israel camp. Israel is fighting back against a murderous terrorist group that carried out mass slaughter.”

Mr. Gallant said on Sunday that Mr. Yaalon’s statements were “a lie that aids our enemy and harms Israel.”

The Israeli military “acted according to the highest standards that can be applied in the complex and difficult war that was imposed on us,” Mr. Gallant said in a post on social media. “The instructions and commands were always given in accordance with the law.”

Israel’s communications minister, Shlomo Karhi, said Mr. Yaalon “crossed all the red lines.” while Tally Gotliv, a firebrand Likud lawmaker, called him “worse than our biggest enemies.”

Israel has called on Palestinians from the northernmost reaches of Gaza to evacuate on several occasions since the war began last fall, including in the first week of the conflict and again in October. Tens of thousands of people have heeded those warnings and fled, but many are believed to have remained in the area, either because they cannot or do not want to leave.

Mr. Yaalon’s statements were striking because they come at a time in which Israelis from across the political spectrum have united in their opposition to the I.C.C.’s issuing of the warrants for Mr. Netanyahu and Mr. Gallant.

They were also unusual because Israelis and their leaders — like people in many countries — tend to rally around the troops during a time of war. Criticism by former Israeli officials of the war has tended to focus on strategy or whether to agree a cease-fire with Hamas, not the military’s conduct veering into potential war crimes.

Of four former senior Israeli security officials contacted by The Times on Sunday, only one agreed to comment.

Ami Ayalon, the former director of the Shin Bet domestic intelligence agency who has criticized Mr. Netanyahu in the past, said he wasn’t sure whether Israel’s actions in Gaza met the legal definition of “ethnic cleansing.” But he described the Israeli government’s policy directives for the military as “immoral and unjust,” saying they could expose commanders and soldiers to prosecution by the International Criminal Court.

In recent months, aid organizations and world leaders, including President Biden, have warned of a humanitarian catastrophe in North Gaza. During that time, Israel has allowed little humanitarian aid to enter. Last month, Israel banned imports of commercial goods, saying that Hamas was benefiting from their sale. North Gaza is the northernmost of Gaza’s five governorates.

Israeli officials have said that Palestinians from North Gaza will be able to return to their homes after the war. Mr. Netanyahu has dismissed the idea of building settlements in Gaza, but hard-line members of his right-wing coalition have advocated for it.

Some Israelis worry that Mr. Netanyahu’s indecision about plans for post-war Gaza could result in a long-term occupation of the enclave, leaving open the possibility for right-wing members of the coalition to advance their ambitions to build settlements.

Some Palestinians from Gaza also took note of Mr. Yaalon’s comments.

Akram Atallah, a Palestinian columnist originally from Jabaliya, said he considered Mr. Yaalon’s remarks to be “extremely important.”

“This remark strengthens the Palestinian narrative of what is happening in Gaza,” he said. “And it isn’t coming from an Arab official or a sympathetic member of the international community. It’s coming from someone who was a general at the top of the Israeli system.”