Authorities in Minneapolis are trying to determine what drove a 23-year-old to open fire at Annunciation Catholic Church on Wednesday morning, killing two children, 8 and 10, and injuring 17 others before taking his own life.

Police said on Thursday that they are examining the suspect’s online videos and writings, which included depictions of weapons and explosives, drawings of the church, and diary-style recordings referencing violence. Minneapolis police chief Brian O’Hara also confirmed the firearms used were legally purchased.

The victims ranged from young children to elderly parishioners, with most survivors expected to recover. Officials noted that some church doors had been barricaded from the outside, suggesting the attack was planned in advance.