The shooter who killed two Catholic school students and wounded more than a dozen youngsters sitting in the pews of a Minneapolis church once attended the same school and was “obsessed” with the idea of killing children, authorities said Thursday.

The shooter, identified as 23-year-old Robin Westman, fired 116 rifle rounds through stained-glass windows while the children celebrated Mass during the first week of classes at the Annunciation Catholic School, said Minneapolis police Chief Brian O’Hara .

“It is very clear that this shooter had the intention to terrorize those innocent children,” O’Hara said.

Acting U.S. Attorney Joe Thompson said videos and writings the shooter left behind show that the shooter "expressed hate towards almost every group imaginable.”

The only group Westman did not hate was “mass murderers and shooters,” Thompson said.

Investigators recovered hundreds of pieces of evidence from the church and three residences, the police chief said. They found more writings from the suspect, but no additional firearms or a clear motive for the attack on the church the shooter once attended.

AP video by Mark Vancleave