Joachim Hermann says two phones, multiple SIM cards and a laptop were found with the body of the 27-year-old asylum seeker or at his accommodation.
The man threatened a "revenge attack" on Germans in the video, he said.
Fifteen people were injured.
Bavarian authorities said that the bomb was constructed in such a way that it was clearly meant to kill as many people as possible.
Further bomb-making equipment was found at the asylum seeker accommodation where the man was living, including petrol, hydrogen peroxide and batteries, they added.
A detailed analysis of the content of the videos was ongoing, Mr Hermann said.
"I think it is unquestionable that it is a terror attack with corresponding Islamist convictions of the perpetrator," he said.
Read the full article here.