The person who the police say opened fire on a Catholic school in Minneapolis on Wednesday appears to have known the school well.

Robin W. Westman, who officials said strafed the church through the stained glass windows, killing two children, was believed to have once attended the school at Annunciation Catholic Church, according to a law enforcement official with knowledge of the investigation.

Her mother, Mary Grace Westman, worked in the business office of the church for five years before retiring in 2021. And in a video posted on social media, the suspect showed a hand-drawn rendering of the Annunciation interior.

And Ms. Westman, armed with three weapons, seemed to choose the time carefully. She barricaded the doors during the first all-school mass of the academic year, the police said.

But it is hard to fathom what drove Ms. Westman to attack before killing herself, despite the dark and violent writings and videos she left behind.

The attack killed an 8-year-old and a 10-year-old in the pews and injured 17 others, according to the Minneapolis police chief, Brian O’Hara. The three weapons were purchased legally, police officials said.