How unusual is it to charge parents in school shootings?

Guns used in U.S. school shootings have often come from the homes of young perpetrators, but parents are rarely charged for the violence that occurs, experts say.

That's what makes the case against Ethan Crumbley's parents uncommon, following the fatal shooting of four students at Oxford High School in southeastern Michigan.

Oakland County prosecutor Karen McDonald said Jennifer and James Crumbley ignored opportunities to intervene, just a few hours before the bloodshed.

They're charged with four counts of involuntary manslaughter, while Ethan, 15, is charged as an adult with murder, terrorism and other crimes.

The Crumbley parents, who were taken into custody early Saturday, and their lawyers haven't commented on the shooting or the charges.