The parents of a 15-year-old accused of killing four students and wounding seven other people at a Michigan high school have been charged with involuntary manslaughter, according to court documents.

Driving the news: James and Jennifer Crumbley were each charged with four counts of involuntary manslaughter.

Their son, Ethan Crumbley, faces two dozen charges, including murder and terrorism, for the shooting at Oxford High School. He will be charged as an adult.

A 15-year-old sophomore opened fire at his Michigan high school, killing three students, including a 16-year-old boy who died in a deputy’s patrol car on the way to a hospital, authorities said.

Eight other people were wounded, some critically, including a 14-year-old girl who was placed on a ventilator after surgery. Investigators were still trying to determine a motive for the shooting Tuesday at Oxford High School, located in a community of about 22,000 people roughly 30 miles (48 kilometers) north of Detroit, Oakland County Sheriff Michael Bouchard said.

Three Michigan high school students were shot and killed on Tuesday, including a "hero" 16-year-old, and eight other people were injured in a school shooting at Oxford High School. 

Oakland County Sheriff Michael Bouchard identified the three students who died as 16-year-old Tate Myre, 14-year-old Hana St. Juliana, and 17-year-old Madisyn Baldwin.

Bouchard said that Myre died inside a patrol car as deputies rushed him to a hospital. 

The terror at Michigan's Oxford High School began with sounds of gunfire, yelling and an urgent message on the loudspeaker.

Then frightened students barricaded doors, phoned for help and picked up anything they could grab in case they needed to fight back.

Senior Aiden Page was in a classroom when he heard two gunshots Tuesday afternoon -- part of a rampage in which authorities say a sophomore killed three students and injured eight other people.