The involuntary manslaughter trial of Jennifer Crumbley is believed to be the first to target a parent of a school shooter.
It's a case that experts say breaks new legal ground and one which gun safety experts hopes can serve as a wake-up call for parents around the U.S. to better secure weapons in their homes.
Jennifer and her husband James each face four counts of manslaughter.
Their son Ethan, who was 15 at the time of the shooting, was last month sentenced to life in prison without parole.
Here's state prosecutor Marc Keast's opening statement on Thursday:
"The evidence will that prove by the time this gun was bought the school shooter was in a downward spiral that had begun months before. The evidence will also show you that Jennifer Crumbly was aware of that. Despite her knowledge of his deteriorating mental crisis, despite her knowledge of his growing social isolation, despite the fact that it is illegal for a 15-year-old to walk into a gun store and walk out with a handgun by himself, this gun was gifted."
Crumbley's defense attorney, said her client "did not have it on her radar" that her son was mentally disturbed.
On the day of the shooting, teachers discovered disturbing and violent drawings from Ethan.
The drawings included a handgun, a bleeding figure, and the words "The thoughts won't stop - help me."
Prosecutors have said the Crumbleys were called to school before the shooting happened, and were told Ethan needed counselling.
The parents resisted the idea of taking their son home, and did not search his backpack nor ask him about the gun.
Unlike some states, Michigan does not legally require gun owners to keep their firearms secured from children.
Michigan also bans those under 18 years of age from buying or possessing firearms, except in cases such as hunting with a license and a supervising adult.
