Jurors found James Crumbley guilty on all four counts of involuntary manslaughter over his teenage son's mass shooting at Michigan's Oxford High School in November 2021.
Four students died when Ethan Crumbley, then 15, opened fire that day. Six other students and a teacher were wounded.
Thursday's verdict came roughly a month after James Crumbley's wife, Jennifer, was convicted on four identical counts of involuntary manslaughter.
The charges against James Crumbley and his wife are thought to be precedent- setting — the first time prosecutors had sought to hold parents of a mass school shooter criminally responsible.
Prosecutors maintained throughout James Crumbley's trial that he should have reasonably foreseen that his son was troubled and primed for violence.
Instead, said Oakland County Prosecutor Karen McDonald in her closing argument, Crumbley bought his son a gun and failed to lock it away from him. To make her point, McDonald took the murder weapon from a courtroom table and threaded a cable lock through it, a process that took about 10 seconds.
She called it one of the "tragically small" steps Crumbley could have taken that would have kept four students at Oxford High School alive.
The prosecution also extensively cited entries Crumbley's son made in a journal and messages he texted to a friend.
In one message, he complained that voices in his head were getting louder but his father refused to take him to a doctor. In a journal entry he wrote, "The shooting is tomorrow. I have access to the gun and the ammo. I'm going to prison for life and many people have about one day left to live."
Prosecutors said the teen made it very clear what he planned to do by drawing a troubling picture on a math test along with scrawled phrases like "Help me" and "Blood everywhere."
McDonald noted Oxford school officials showed the Crumbleys the drawing at a meeting hours before the shooting began.
"How many times does this kid have to say it? He writes the words 'Help me' on a piece of paper! And his parent, James Crumbley, is called to the school and what does he say? He says, 'We gotta go to work.' "
Throughout the trial the defense painted James Crumbley as a caring parent who had no idea his son was descending into a world of hallucinations and potential violence.
In her closing arguments, Defense Attorney Mariell Lehman told jurors there was no proof the teen was handling firearms by himself in the Crumbley household. She also said a school counselor had examined the same bloody drawing shown to the Crumbleys and decided their son did not pose a threat to himself or anyone else.
"We know what happened within a couple hours of this meeting. It's easy to look back and say that different decisions could have been made," Lehman said.
Crumbley, dressed in a suit and wearing headphones to help with his hearing, presented a mostly calm figure in court.
He did look away from courtroom monitors, swallowed hard and wiped his nose with a tissue as the prosecution played graphic security video of the bloody shooting.
A different side of Crumbley emerged midway through the trial, when his communications from jail were severely restricted.
Law enforcement officials confirmed that Crumbley had made threats while incarcerated over a jailhouse phone and in an electronic message.
Jurors never saw that view of Crumbley and, unlike his wife a month earlier, James Crumbley never took the stand in his own defense.
James and Jennifer Crumbley will be sentenced early next month.
Both face a maximum of 15 years in prison.
Ethan Crumbley did not testify in either trial for his parents. He pleaded guilty in October 2022 to the killings and was sentenced to life in prison without parole.
