Jurors found James Crumbley guilty after prosecutors successfully argued that he did not keep the murder weapon locked away from his son.
Ethan Crumbley was sentenced to life without parole for using that gun to kill four classmates in 2021.
The Detroit Free Press reports the jury foreman believes James Crumbley had many chances to secure the firearm...but never did.
The foreman, who wants to remain anonymous because of privacy concerns, reportedly said other jurors thought Crumbley should have realized his son was troubled, especially after he was called to the school the day of the shooting to view a violent picture the teen drew.
His mother, Jennifer Crumbley, was convicted a month ago on identical charges.
Both Crumbleys face sentencing on April Ninth.
