International

The Islamic State claimed responsibility on Tuesday after a 17-year-old Afghan who came to Germany as a migrant attacked passengers on a regional train with an ax before he was killed by the police, a development that is likely to intensify fears that the huge influx of migrants poses a security threat.

The announcement, in a bulletin issued in Arabic and English via its Amaq News Agency, came after German authorities said that investigators had found a hand-drawn flag of the Islamic State, also known as ISIS or ISIL, in the room of the Afghan teenager, along with notes in Pashto indicating that he might have been self-radicalized.

Approximately 1.5 million migrants have applied for asylum in Germany in the past 18 months, but the enthusiastic welcome they initially received has given way to one of increasing concern as the euphoria of generosity is overshadowed by the difficult reality of integration.

Although the flow has slowed, Chancellor Angela Merkel has been under increasing pressure, especially after North African migrants were linked to hundreds of sexual assaults in Cologne on New Year’s Eve.

In a statement that used language similar to an announcement after an attack on Friday in Nice, France, the Amaq News Agency said that the ax assault had been committed by an “Islamic State soldier.” The statement added that he had acted in response to a call to target coalition countries fighting the militant group.

The teenager, who has not yet been identified, arrived from Afghanistan without his parents to apply for asylum last year. The authorities believed that he carried out the assault alone Monday night, but that has not been confirmed, said Joachim Herrmann, interior minister of the southern state of Bavaria, where the attack occurred.

The authorities were trying to determine what motivated the teenager to carry out the attack, which left at least four people wounded, and whether he had direct contact with operatives of the Islamic State or was simply motivated by online propaganda.

“The first emergency call to the police from a witness in the train said that he had shouted ‘Allahu akbar,’ ” Mr. Herrmann told the public broadcaster ZDF. “In searching the room where he last lived, a hand-drawn I.S. flag was found.”

“This must now all be put together in like a big mosaic,” he said, “to figure out what his motivation was and the extent to which he really belonged in an Islamic movement, or whether he became self-radicalized very recently.”

Alexander Gross, superintendent criminal detective of the Bavarian State Office of Criminal Investigations, said that his office was aware of the Islamic State claim but could in no way comment on its veracity.

“Just because I.S. is claiming this attack does not mean there is anything to it,” Mr. Gross said. “Right now, we have to examine in great detail who he knew and with whom he was in contact in order to create a complete picture” of what motivated him.

After the teenager lunged at several people, a passenger pulled the emergency brake on the train, which was traveling to Würzburg from Treuchtlingen, the federal police in Würzburg said.

The teenager then fled the stopped car into a district of Würzburg, where he encountered police officers, including special forces. He lunged at the officers, who responded by opening fire on the young man and killing him, Mr. Herrmann said.

Four of the victims were from the same Hong Kong family, and the territory’s leader, Leung Chun-ying, condemned the attack on Tuesday. Two are in critical condition, Mr. Herrmann said. Mr. Leung sent representatives from Hong Kong and its Berlin trade office to Würzburg to follow up and provide assistance.

More than 60,000 unaccompanied minors were registered as asylum seekers in Germany last year, and more than half of the approximately 15,000 in Bavaria were from Afghanistan. Under German law, they cannot be extradited until they are 18 and are granted the same rights, such as access to education and financial support, as German juveniles who live on their own.

Many of the young refugees are in group homes, but others have been placed in foster families. Mr. Herrmann said the attacker had most recently lived with a family near Würzburg.

Germany has not experienced attacks on the same scale as Belgium or France, but it remains on edge amid threats on social media by Islamist extremists and the repeated targeting of its European neighbors. Several plots have been foiled by the police.

In May, a 27-year-old German killed one man and wounded three others with a knife while shouting “Allahu akbar” on a commuter train in a suburb of Munich. After questioning him, the authorities said that he had no known links to extremist groups and that they believed he was mentally disturbed.

Fears that terrorists may have entered the country among the hundreds of thousands of migrants have been running high. With the attack coming days after a Tunisian man drove a truck down a street packed with pedestrians on Bastille Day in Nice, the assault Monday night could have wider political ramifications.