Were they or weren't they? That is a provable/disprovable assertion, and I think I would have researched it further before making any statement about its probability if I were you.
In addition, I would like to see some compelling evidence for your earlier claim that most murderers are from "at least nominally" (nice wiggle room) Christian families.
The Columbine killers were an anomaly.
These Muslim boys are a dime a dozen.