Violent drawings made by Ethan Crumbley were seen by his parents prior to the Michigan school shooting, the court has heard.
Crumbley is accused of fatally shooting four students and injuring seven at Oxford High School in Michigan on November 30.
He was charged with four counts of first-degree murder, one count of terrorism causing death, seven counts of assault with intent to murder, and 12 counts of possession of a firearm in the commission of a felony. The teenager pleaded not guilty to the charges.
His parents Jennifer and James Crumbley have been charged with four counts of involuntary manslaughter and also pleaded not guilty.
The 15-year-old is said to have doodled drawings of a gun and a bullet on a school assignment about congruent triangles, which was spotted by a teacher on the morning of the deadly shooting at Oxford High School on November 30.
Words reading "thoughts won't stop, help me", "the world is dead" and "my life is useless" were also written on the note, however, when confronted about the drawings Crumbley claimed they were for a "video game he was designing", according to a letter written by Oxford Community Schools Superintendent Tim Thorne.
Prosecutors filed the drawings as part of documents on Thursday, December 23, alleging the teenager had scribbled over some parts and added mock cheerful phrases after being caught out.
Images of the assignment also revealed a drawing of a person apparently lying in a pool of blood, while in later edits Crumbley is said to have added notes reading "video game this is", "were [sic] all friends here" and "OHS Rocks!"
The filing, cited by New York Post, reads: "What is novel about this case is that [the parents] were made aware, in graphic form, of the serious risk posed by their son prior to the shooting.
"This is not a case of hindsight, where parents later wish they could have done something. These parents could have done something. Despite having all this knowledge, [defendants] failed to take even the simplest action that would have prevented the massacre.
"All they had to do was tell the school that they recently purchased a gun for their son, ask him where the gun was, open his backpack, or just take him out."
