A teenager pleaded guilty Monday to terrorism and first-degree murder in a Michigan school shooting that killed four students and put an extraordinary focus on the boy's home life and the alleged role of his parents in the tragedy.

Ethan Crumbley pleaded guilty to all 24 charges, nearly a year after the attack at Oxford High School in southeastern Michigan. In the gallery, some relatives of the victims were weeping as assistant prosecutor Marc Keast described the crimes. The teenager repeatedly said 'yes' to each detail.

The prosecutor's office said no deals were made ahead of Monday's plea. A first-degree murder conviction typically brings an automatic life prison sentence in Michigan, but teenagers are entitled to a hearing at which their lawyer can argue for a shorter term and an opportunity for parole.

The teenager withdrew his intent to pursue an insanity defense as part of the plea, and repeatedly acknowledged that he understood the potential penalties.

Ethan, now 16, had no discipline issues at the school, roughly 30 miles (50 kilometers) north of Detroit, but his behavior earlier on the day of the mass shooting raised flags.

A teacher had discovered a drawing with a gun pointing at the words: "The thoughts won't stop. Help me." There was an image of a bullet with the message: "Blood everywhere."

James and Jennifer Crumbley declined to take their son home on Nov. 30 but were told to get him into counseling within 48 hours, according to investigators.

Ethan had brought a 9mm Sig Sauer handgun and 50 rounds of ammunition to school in his backpack that day and subsequently fired it at fellow students. Deputies rushed in and captured him within minutes.

A day earlier, a teacher had seen Ethan searching for ammunition on his phone. The school contacted Jennifer Crumbley, who told her son in a text message: "Lol. I'm not mad at you. You have to learn not to get caught," the prosecutor's office said.

Separately, the elder Crumbleys are facing involuntary manslaughter charges. They are accused of making a gun accessible to Ethan and ignoring his need for mental health treatment. Parents have rarely been charged in school shootings, though the guns used commonly come from the home of a parent or close relative.

Prosecutors earlier this year disclosed that Ethan had hallucinations about demons and was fascinated by guns and Nazi propaganda.

"Put simply, they created an environment in which their son's violent tendencies flourished. They were aware their son was troubled, and then they bought him a gun," prosecutors said in a court filing.

The Crumbleys said they were unaware of Ethan's plan to commit a school shooting. They also dispute that the gun was easy to grab at home.

Madisyn Baldwin, Tate Myre, Hana St. Juliana and Justin Shilling were killed, while six students and a teacher were wounded. In addition to the counts of first-degree murder and terrorism causing death, Ethan was charged with seven counts of assault with intent to murder and 12 counts of possessing a firearm in the commission of a felony.