An attack by a man armed with a knife that left three dead at a local festival in a western German city is being treated as terrorism, with possible links to the Islamic State, German prosecutors said on Sunday

The suspect is a 26-year-old man from Syria who was living in a refugee residence less than a few hundred yards from where the attack took place in the city of Solingen, the police said on Sunday. The man, wearing bloodstained clothes, approached a police car and gave himself up after 11 p.m. Saturday, according to law enforcement officials.

The police said the assailant aimed for his victims’ necks to inflict as much damage as possible.

On Sunday afternoon, the federal prosecutor’s office said it believed the suspect, identified only as Issa Al H. in keeping with strict German privacy rules, had joined the Islamic State. Officials are also investigating him on possible charges of murder and attempted murder, though so far no official charges have been filed.

The suspect “shared the ideology” of the terrorist organization and “joined the group at an undeterminable” time before Friday’s attack, Ines Peterson, spokeswoman for the federal prosecutor, said in a statement Sunday.

The Islamic State took responsibility for the attack, praising the attacker as a “soldier of the Islamic State,” according to Site Intelligence Group, which tracks extremist organizations.

On Sunday, the site of the attack was still cordoned off and guarded by the police, but mourners gathered at an adjacent church to light candles, lay flowers and leave condolence messages on a large white banner.

“This is the first year we skipped the festival and then this happens,” said Tanya Zastrov, 50, a resident of Solingen, referring to the town’s 650th birthday festival that the attack so tragically upended. “I’m shocked and speechless,” she added.

Solingen, a city of over 150,000, is ethnically diverse and some condolence messages were written in other languages besides German, such as Turkish and Greek. During its boom years in the 1960s and 1970s, the city relied on foreign workers to fill its production plants.

“If someone opens the door for you, then you should be grateful,” said Ahmet Kalayci, 58, who moved to Solingen from Turkey when he was 16. He added: “Then you should behave.”

The attack on Friday occurred less than three months after a similar attack by an Afghan refugee in Manheim, another ethnically diverse city in the country’s west, about 130 miles south of Solingen. In that case, an Afghan refugee attacked an anti-immigrant rally with a knife and killed a police officer who tried to intervene.

Earlier this month, three Taylor Swift tour dates were canceled in Vienna after U.S. intelligence officials warned their Austrian counterparts that two teenagers were planning on attacking the crowds gathered for the event. The main suspect in that plot had radicalized online, officials said, and had filmed himself swearing allegiance to the Islamic State.

The far-right Alternative for Germany party, which has campaigned largely on an anti-immigrant platform and is poised to make significant gains in three state elections next month, jumped on the news of the Solingen attack. Even before the identity of the attacker was confirmed by the police, one of its leaders called for changes to “migration and security policy.”

The authorities had earlier arrested two people who were later determined unlikely to have been the actual attackers, Herbert Reul, the state interior minister of North Rhine-Westphalia, where Solingen is, said in an interview on Saturday with a German broadcaster, ARD.

A 15-year-old boy, who was arrested early Saturday, is being investigated for not having alerted the police when he learned about imminent plans to attack, prosecutors said. A man arrested by a heavily armed police unit on Saturday evening in the refugee housing facility where the main suspect also lived is being treated as a witness, the police and Mr. Reul said.

On Saturday, Solingen’s mayor, the state governor and other political leaders gathered on a downtown square several hundred yards from where the attack took place to mourn the victims. It was an eerie repeat of a similar impromptu service held in Mannheim after the attack there.

On Sunday, which was supposed to be the final day of joyous festival celebrating a city best known for making knives and scissors, the downtown was mostly deserted.

On the boulevard in front of the presidential palace in Taipei this weekend, Taiwan’s worst nightmare was unfolding in front of film crews. A crowd of actors and extras portrayed one kind of chaos that might come with a Chinese invasion: a protest descending into violence and bloodshed.

The scene being shot was for “Zero Day,” a new Taiwanese television drama series that depicts an effort by China to take over the democratically governed island. Beijing has long claimed Taiwan as its territory and urged it to peacefully accept China’s sovereignty. The Chinese leader, Xi Jinping, has said he would not rule out using force to absorb it.

“Zero Day” will not air until next year, but it has already set off heated debates in Taiwan, after the release of a trailer. Supporters of the series say it could encourage a much-needed conversation about the threat that China poses. Critics have denounced it as scaremongering.

Cheng Hsin-mei, the producer of “Zero Day,” said she wanted to jolt Taiwanese people out of what she sees as widespread complacency and reticence about the possibility of war.

“How everyone would really face up to a war, how you would confront that possibility — nobody’s actually talking about that,” Ms. Cheng, who is also a main scriptwriter for the series, said in an interview. “I want to talk about it, because I think it’s the biggest fear in each Taiwanese person’s heart.”

The show “Zero Day,” a 10-episode series, imagines how China could mount a blockade around Taiwan, then try to overrun the island, a possibility that many experts see as increasingly plausible.

The drama follows a Taiwanese television presenter, an online celebrity, a (fictional) president and president-elect and other characters as they confront a weeklong Chinese campaign. The blockade leads to shortages on the island, looting and financial meltdown. Foreigners are evacuated. Finally, as Chinese troops land, fighting ensues. The characters wrestle with whether to flee or stay, and whether to collaborate or resist. The tone is somber, to judge from the show’s 17-minute trailer, which was issued online before the series had finished shooting.

“Nothing in reality is black and white,” said Janet Hsieh, who plays a Taiwanese president-elect in the series. “It is highlighting the complication of situations, of families, of a lot of the political things that are happening.”

Despite the extensive policy research that has been done about the risk of an invasion of Taiwan, until now, no movie or television drama has explored these questions for a wider public, apparently because of the topic’s political contentiousness.

Some Taiwanese actors turned down roles in the show, Ms. Cheng said, out of concern that they would be blacklisted by China or lose sponsors. The owners of some buildings or sites pulled out of agreements for scenes to be shot on their premises, apparently worried about courting controversy.

Critics, mostly from Taiwan’s opposition, said “Zero Day” amounted to propaganda for the governing Democratic Progressive Party, which firmly rejects Beijing’s claims to the island. Politicians from the opposition Nationalist Party, which argues for stronger ties with Beijing, pointed out that Taiwan’s Ministry of Culture and a government-linked fund invested in the production, and that scenes were filmed at Taiwanese military sites and inside the presidential palace.

“This is totally using the power of the state for Democratic Progressive Party propaganda,” Jaw Shaw-kong, a vice-presidential candidate for the Nationalists in this year’s election, told reporters. “It’s tantamount to an election ad.”

Lo Ging-zim, one of the 10 directors involved in the series, each directing one episode, said it was normal for Taiwanese television and film productions, including comedies and horror movies, to win some government funding. He said the government had not sought to influence the direction of the drama.

Mr. Lo said he was inspired to join the “Zero Day” project after Russia’s invasion of Ukraine. The characters in the series will grapple with measures that China may use to destabilize Taiwan, like flooding the internet with disinformation, Mr. Lo said. In the trailer, as the Chinese blockade begins, rumors claiming that the president of Taiwan has fled are shared on social media.

“In a real-world scenario, Beijing will wage political, psychological and legal warfare against Taiwan to sow divisions and confusion among Taiwan’s public,” said Brian Hart from the Center for Strategic and International Studies in Washington and an author of a new study of how China could try to blockade Taiwan. “I think that is a big part of what this show is trying to emphasize to viewers. That’s important because deterring and resisting Chinese attacks requires more than just military capabilities.”

The scene recently shot in front of the presidential palace dramatized one way that might happen. As a crowd of Taiwanese protesters calls for peaceful compromise with Beijing, pro-China infiltrators provoke divisions, setting off fights that prompt the police to step in.

“If there was too much heroics, that would a bit too far into fantasy,” said the director of the episode, Wu Zi-en. “It would be a bit too out of step with reality.”