A video has been found on the Ansbach bomber's phone showing him pledging allegiance to the leader of Isis, Bavaria's top security official has said.
Joachim Herrmann said according to an initial translation of the Arabic-language video the 27-year-old man announced a "revenge" attack against Germany.
"A provisional translation by an interpreter shows that he expressly announces, in the name of Allah, and testifying his allegiance to Abu Bakr al-Baghdadi, a famous Islamist leader, an act of revenge against the Germans because they're getting in the way of Islam," he said at a news conference.
Mr Herrmann added: "I think that after this video there's no doubt that the attack was a terrorist attack with an Islamist background."
Isis has claimed responsibility for the attack through its Amaq News Agency.
The 27-year-old failed asylum seeker, who has been named in German media as Mohammad Daleel, detonated a device outside a crowded music festival, killing himself and injuring 15 others.
His rucksack contained sharp pieces of metal. Four of those injured are in a serious condition.
Officials also said police found violent videos and bomb-making material at his home.
Mr Herrmann said officers discovered videos with "Salafist content" on storage devices seized at the Syrian man's home.
He told reporters police also found gasoline, chemicals and other material that could be used to make a bomb.
Daleel had been living in Germany for two years and had received psychiatric treatment, including twice for attempted suicide, authorities said.
Last year his application or asylum was rejected by German authorities, though due to the Syrian civil war he had been allowed to stay in the country.
He was facing depotation to Bulgaria, a German interior ministry spokesman said.
"Syrians cannot at the moment be deported to Syria at the moment, but that doesn't mean that Syrians overall cannot be deported," Tobias Plate told a regular government news conference.
"The Syrian in Ansbach was facing deportation and this was to Bulgaria," he added.
On Sunday, a 21-year-old Syrian refugee was arrested after killing a pregnant woman and wounding two people with a machete in the southwestern city of Reutlingen, near Stuttgart.
Additional reporting by agencies
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