Court documents, filed by his attorneys on Thursday, state that the 15-year-old suspected gunman “intends to assert the defense of insanity at the time of the alleged offense and gives notice of his intention to claim such a defense”.
Mr Crumbley will now be required to undergo a psychiatric exam ahead of a trial.
The teenager is charged as an adult with 24 counts including four counts of first-degree murder and one count of terrorism over the 30 November mass shooting at Oxford High School, after he allegedly took a handgun to school and murdered his classmates.
Four students were killed in the attack – Hana St. Juliana, 14, Tate Myre, 16, Madisyn Baldwin, 17, and Justin Shilling, 17 – and seven other people were wounded in what was America’s deadliest school shooting since the Parkland massacre in 2018.
Mr Crumbley’s parents James and Jennifer Crumbley are also charged with four counts of involuntary manslaughter after they allegedly bought their son the firearm used in the attack as an early Christmas present and left it accessible to him despite multiple warning signs about his disturbing behaviour.
News of their son’s intention to plead insanity comes the same day a new lawsuit was filed against the Crumbleys and school officials and just days after Oxford High School reopened for the first time on Monday in the wake of the mass shooting.
Tate Myre’s family and the families of survivors are suing school officials and Mr Crumbley’s parents for negligence for at least $25,000.
Prosecutors said Mr Crumbley exited a bathroom in the school with a firearm which his parents had bought for him just days earlier.
The suspect then allegedly opened fire on his fellow students in the hallway as they were changing classes.
He surrendered to law enforcement officers as they arrived on the scene.
The motive for the mass shooting is not clear.
However, prosecutors have revealed a disturbing pattern of behaviour in the days and weeks leading up to the attack.
One day before the massacre, a teacher found Mr Crumbley searching for ammunition on his phone, according to prosecutors.
School staff attempted to alert his parents James and Jennifer Crumbley to their concerns but said messages went ignored and the 15-year-old was allowed to remain in the school building.
On the morning of the shooting, officials said another teacher then found a drawing on the teenager’s desk of a handgun, a bullet and a person being shot.
“The thoughts won’t stop, help me” and “blood everywhere” were scrawled on the note.
That time, his parents were called in to the school and a meeting was held with them, their son and school officials.
The teenager claimed that the drawings were simply designs for a video game, according to school officials.
His parents fought to have him returned to class, prosecutors said, and were told to get him counselling within 48 hours. Just a few hours later, he allegedly opened fire in the school.
A lawsuit brought by the families of survivors also alleges that Mr Crumbley took bullets to the school the day before the attack and showed them off to his classmates.
He also brought a bird’s head to school in a mason jar filled with yellow liquid and left it in the bathrooms three weeks before the 30 November massacre, the suit claims.
Mr Crumbley initially pleaded not guilty to all charges at an arraignment in December.
Prosecutor Karen McDonald has previously said she does not believe the suspect’s mental state will impact his ability to stand trial.
Days after the shooting, prosecutors took the unprecedented step to charge the teenager’s parents over the mass shooting.
The couple allegedly bought the gun used in the attack – a Sig Sauer 9mm, model SP 2022 – for their son as a Christmas present four days earlier on Black Friday and left the gun accessible to him.
The 15-year-old allegedly boasted about “my new beauty” on his Instagram account, prosecutors said.
His parents then failed to remove him from the school or to take any action despite the warning signs about his behaviour, said prosecutors.
Yet, when news of the shooting spread, Jennifer Crumbley allegedly texted her son “Ethan, don’t do it”, officials said.
In a bizarre turn of events, the couple then appeared to go on the run after prosecutors announced charges against them.
They were tracked down and arrested hiding out in a warehouse in Detroit, having allegedly withdrawn $6,600 in cash, sold their horses, and bought four burner phones.
They have both pleaded not guilty to the charges.