The Syrian who blew himself up in southern Germany, wounding 15 people, had pledged allegiance to Islamic State on a video found on his mobile phone, the Bavarian Interior Minister said on Monday.
On searching the bomber's room, Nuremberg police found diesel, hydrochloric acid, alcohol, batteries, paint thinner and pebbles - the same materials used in the bomb - and computer images and film clips linked to the militant group, they said.
"A provisional translation by an interpreter shows that he expressly announces, in the name of Allah, and testifying his allegiance to [Islamic State leader] Abu Bakr al-Baghdadi ... an act of revenge against the Germans because they're getting in the way of Islam," Bavarian Interior Minister Joachim Herrmann told a news conference.
"I think that after this video there's no doubt that the attack was a terrorist attack with an Islamist background."
Islamic State claimed responsibility for the bombing, according to Amaq, a news agency that supports Islamic State.
"There was also a laptop that showed pictures and film sequences that glorify violence and are unequivocally linked to Islamic State," he told a news conference.
The attack on Sunday, outside a music festival in Ansbach, a town of 40,000 people southwest of Nuremberg that has a US Army base, was the fourth act of violence by men of Middle Eastern or Asian origin against German civilians in a week.
Fertinger added that police had arrested a suspect who knew the bomber. They were trying to find out if the attacker had help making the bomb and whether it exploded prematurely, which could suggest he wanted to kill as many people as possible.
The incident, after three other attacks since 18 July that left 10 people dead and dozens injured, will fuel growing public unease about Chancellor Angela Merkel's open-door refugee policy. More than a million migrants entered Germany over the past year, many fleeing war in Afghanistan, Syria and Iraq.
Germany's federal and state security authorities have more than 400 leads on fighters or members of Islamist organizations among refugees in the country, the BKA federal police said.