A bloody rampage by an ax-and-knife wielding Afghan refugee has revived concerns in Germany about the country's ability to cope with an influx of migrants over the past year, some of whom may harbor violent hatred toward the west.

Riaz Khan Ahmadzai, 17, attacked tourists from Hong Kong while traveling on a commuter train near the Bavarian town of Wuerzburg on Monday night, wounding four on board and a local German dog walker outside as he fled. Two of them remained in critical condition Thursday.

The Islamic State group claimed responsibility for the attack and posted a video in which the teen waved a knife and referred to himself as a "soldier of the Islamic State". Authorities, however, have said that the attacker, who was shot and killed after lunging at police with his weapons, likely acted alone after becoming radicalized.

The nationalist Alternative for Germany, a political party that has surged in popularity on an anti-immigration platform, has seized on the train attack, claiming it was proof that Chancellor Angela Merkel's policy of opening German borders to refugees had failed.

Georg Pazderski, a member of AfD's national leadership, said the attack showed Germany was "sitting on a dangerous time bomb" in the shape of thousands of young refugees.

Germany's justice minister cautioned against jumping to hasty conclusions after the attack.

"It would be a big mistake to prematurely declare that integration (of migrants) has failed," Heiko Maas told German daily Bild. He noted that most of the recent extremist attacks in Europe were carried out by people born or raised in Europe.

Ahmadzai was one of about 60,000 unaccompanied minors who came into Germany last year amid an unprecedented influx of more than 1 million asylum seekers, mostly from Syria, Iraq and Afghanistan.

Authorities have been providing special care to these minors, many of whom are traumatized by the conflicts in their homelands and by their journey to Europe.

Officials and people who knew Ahmadzai, including his foster parents, struggled to explain how the apparently well-adjusted teenager might have become radicalized.

"He was active on social media, had his page there, but there were no hints of any extremist Islamist thought" until a "cryptic message about the enemies of Islam" posted shortly before the attack, Bavarian criminal police investigator Lothar Koehler said.