The southern German city’s police chief said investigators had found a trove of electronic data and written materials at the suspect’s home suggesting that he had extensively researched shooting sprees before he went on one of his own Friday afternoon. The items recovered included a book by a U.S. academic on school shootings titled “Rampage in the Head: Why Students Kill.”

The killer’s full name has not been disclosed. But a spokesman for the Munich prosecutor’s office said he was an 18-year-old dual Iranian-German national who was born and raised in Munich, and who alternately used the first names David and Ali. He did not have a criminal record, but “may have had a mental disorder,” according to Thomas Steinkraus-Koch, Munich’s prosecutor.

Whatever the killer’s exact motive, he acted alone, police chief Hubertus Andrä said, and his behavior had “nothing to do with immigrants or immigration,” despite speculation to the contrary throughout Friday afternoon and evening as the attack unfolded.