A 26-year-old hair braider named Josephine Owino disappeared one morning last month in the sprawling shantytown of Mukuru Kwa Njenga in Kenya’s capital, Nairobi, after going out suddenly to see someone who had just phoned.

Ms. Owino’s younger sister, Peris Keya, was desperate to find her, and went to three police stations pleading for help. But nothing happened until Ms. Keya said she had a startling dream one night: Her sister appeared, led her up a hill and begged her to search in a pool of water.

Since then, at least 10 sacks with body parts have been fished from an abandoned quarry filled with a thick bed of floating trash, according to the police and human rights activists. The dump was searched only because Ms. Keya, 24, beseeched some local men to help, paying them for the grisly task.

On Monday, the Kenyan police announced that they had arrested a suspected serial killer, who they said had confessed to killing 42 women, including his own wife, in the past two years, and throwing them into the dump.

The suspect’s lawyer accused the police of using torture to extract a confession. And the speed with which the police made the arrest left many Kenyans suspicious. But the police said they had traced their way to the suspect, Collins Jumaisi Khalusha, 33, after doing a forensic analysis of a cellphone belonging to one of the victims.

The discovery of the body parts in the dump — located across the street from a police station — has shocked Kenyans, spreading fear and rumors about who could have committed such grisly murders.

It has also brought intense scrutiny of the Kenyan police, raising questions about how they could have failed to detect or investigate the disappearance of so many women.

Two families in addition to Ms. Keya told The New York Times that for a month, they had been telling the police in Nairobi that their female relatives were missing but got no help.

“They felt nothing for us,” Ms. Keya said of her pleas to the police to look for her sister. As she sat at various police stations for hours, cradling her sister’s toddler who was crying for his mother, “I was shaking with anger,” she said.

An epidemic of gender-based violence and murders of women set off widespread protests in Kenya this year.

The criticism of the police intensified in recent weeks after they were accused of using live bullets to put down even larger demonstrations against tax hikes introduced by President William Ruto. At least 50 people were killed in those demonstrations, the Kenya National Commission on Human Rights said. The police have also been accused of conducting brutal abductions of outspoken activists and this week, of a journalist.

The country’s inspector general of police resigned this month after the uproar over the killings of protesters. The Kenyan police — who have been deployed to bring order to gang-ravaged Haiti — have a long history of impunity, using excessive force and carrying out extrajudicial killings.

Kenya’s police did not respond to repeated requests for comment about the case.

At the dump site in Mukuru, the rancid smell of garbage mixed with the choking smoke from burning trash. Just over 100 yards away, past an empty field and down a muddy alley, the suspect, Mr. Khalusha, had rented one room in a decrepit hovel, residents said.

Neighbors say he mostly kept to himself and sold SIM cards at the nearby market. They said they saw women visiting him, some of whom stayed over the weekend and socialized with other neighbors. But they never witnessed him abusing the women or heard any screams or loud noises from his room.

Vincent Oloo, a neighbor in the building who had known the suspect for almost a year, said it would have been impossible that he could have killed 42 women in such close quarters and gotten away with it.

“Everyone here knows about everyone else’s business,” Mr. Oloo said.

In the frantic search for her sister, Ms. Keya said that at first, she went from one police station to another, sharing her sister’s phone number and identity card.

Days later, the breakthrough came after Ms. Keya’s dream, according to accounts from two of her family members as well as the police. Ms. Keya said she dreamed that her sister woke her up, led her toward a hill, pointed down, and said, “I am inside the water,” adding, “Please get me out. Please find me and bury me.”

The next day, Ms. Keya walked to the only hill nearby, overlooking the disused quarry. A day later, she returned with a friend and asked several young men milling around the site if they could help find a body. The men demanded money, and she agreed to pay them about $15.

Hours later, the men found a sack with a mutilated body inside but demanded more money to lift it out. Ms. Keya, hopeful that the body might be her sister, darted to the police station opposite the dump site. After explaining her ordeal yet again, she returned with several police officers, who watched the young men lift the sack out of the dumpsite.

Three days later, in Mr. Khalusha’s room near the dump site, the police said they found smartphones, women’s underwear, a machete and a pink handbag, among other things. Two other people are also being held, the police said, with one found in possession of the phone of one of the slain women and the other said to have sold it.

Most of the sacks recovered from the dumpsite contained amputated limbs and torsos, according to a government pathologist, Dr. Johansen Oduor. Only one intact body was recovered, Mr. Oduor said. All the identified bodies are women.

The bodies were at various levels of decomposition, making it hard to identify the cause of death for some, Mr. Oduor said. None of the corpses had bullet wounds, he said, but one had been strangled to death. Pathologists have so far identified two bodies through DNA.

They have not yet matched any of the remains to Ms. Keya’s sister.

Police officers at the station closest to the dump site were transferred this week, said Douglas Kanja, the acting inspector general of police. In addition, Kenya’s independent police watchdog said it was investigating whether there was “any police involvement in the deaths, or failure to act to prevent” the murders.

“The police are incompetent in the manner in which they treat complaints from members of the public,” said Hussein Khalid, a lawyer and veteran human rights activist. “It is laxity, it is unprofessionalism. It is really unacceptable.”

The suspect’s lawyer, John Maina Ndegwa, said in an interview that the police had strangled and tortured his client into giving a confession. When he first met him, he said, his client was writhing in pain and needed urgent medical care. When Mr. Ndegwa tried to meet his client on Thursday, he said police officers had blocked him.

“Anybody fearing for his or her life would say whatever his oppressors want to hear,” Mr. Ndegwa said of the confession. The whole case, he said, “negates common sense.”

Families of women who have gone missing say they are awaiting justice and a chance to properly bury their loved ones.

This includes the family of Roseline Akoth Ogongo, a 24-year-old who moved to Nairobi three months ago and worked as a casual laborer. Ms. Ogongo’s brother, Emmanuel Ogongo, said his sister was happy and outgoing and loved posting videos on TikTok. On the morning of June 28, she left home, never to be seen again.

When bodies began surfacing at the dump site last week, the family rushed to the morgue to see if she was among them. As they viewed the mutilated bodies, they recognized a yellow T-shirt she loved to wear. Only her torso lay on the metal table, Mr. Ogongo said.

Later, as the police showed them photos of evidence found in the suspect’s home, Mr. Ogongo said they recognized another item: the pink handbag.

“I couldn’t believe I was looking at it,” Mr. Ogongo said in an interview.

He kept staring at a photo on his phone of his sister’s mutilated body. Every few minutes, he flipped to watch the video of her dancing in the same yellow T-shirt.

“I really miss her,” he said.

The building, once a warehouse, apartments and offices, is a temporary home — with one shower — for 60 adults and children. On the ground floor, rats sprint under plastic chairs and parked baby strollers. The stench of damp clothes and clogged toilets overpowers the strong scents of tomato and spices from the makeshift kitchenettes on upper floors. In the inner courtyard, laughter echoes as children scoop up giggling babies and gently swing them skyward.

This is a so-called squat in Seine-Saint-Denis, a suburban area east of Paris that at one time was an industrial district. Now, it is a place with trendy cafes and high-fashion houses, as well as abandoned factories and spaces like the warehouse, which have become unauthorized housing for homeless people and immigrants.

Mariam Komara, 40, an undocumented immigrant from the Ivory Coast, has lived there since last year. The other day she was getting ready to go to court to argue that she has the right to stay.

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