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.
Prosecutors said Mr Crumbley exited a bathroom in the school with a firearm which his parents had bought for him as an early Christmas present just days earlier.
The suspect then allegedly opened fire on his fellow students in the hallway as students were changing classes.
He surrendered to law enforcement officers as they arrived on the scene.
The motive for the mass shooting - the deadliest in a US school since the Parkland massacre in 2018 - is not clear.
However, prosecutors have revealed a disturbing pattern of behaviour in the days 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 but 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, said prosecutors.
His parents fought to have him returned to class, prosecutors said, and were told to get him counselling within 48 hours.
Mr Crumbley was allowed to return to class and his backpack and locker were not searched.
Just a few hours later, he allegedly opened fire in the school.
The teenager’s parents have also been charged with four counts of involuntary manslaughter over the mass shooting, in what was an unprecedented move by the local prosecutor.
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.
When news of the shooting spread, Jennifer Crumbley allegedly texted her son “Ethan, don’t do it”.
Meanwhile, James Crumbley called 911 to report a gun was missing and to say that he believed his son to be the shooter, officials said.
In a bizarre turn of events, the couple appeared to go on the run after the prosecutors announced charges against them.
They were tracked down and arrested hiding out in a warehouse in Detroit and taken into custody.