Ethan Crumbley, 15, killed four people and injured six more at his school in Oxford, north of Detroit, on November 30, using a gun his parents had bought him days earlier.

After trying to flee, parents James and Jennifer Crumbley, aged 45 and 43, were charged with manslaughter, a charge very rarely used in such cases.

On Tuesday, they appeared handcuffed and with their legs shackled, before a Michigan judge who will decide if there is enough evidence to send them to trial.

Prosecutors called a series of witnesses to the stand who gradually painted a damning picture of the couple, showing how they appeared to have been willfully blind to the many warning signals from their son.

An investigator presented text messages sent by Ethan Crumbley to his mother more than seven months before the tragedy, in which he said he heard noises, saw "demons" and evoked his "paranoia".

"Can you get home now?" he texted his mother one evening when he was in the family home alone. "There is someone at home I think" before adding, "Maybe it's just my paranoia."

Analysis of Jennifer Crumbley's phone showed she didn't answer until the next morning.

During that time, "she was horse-riding," Detective Edward Wagrowski said, showing selfie photos she had taken at the time.

To those around her, she spoke "every day" about riding, but very rarely about her son, limiting herself to evoking a "lonely" child, affected by the death of his dog and the departure of his only friend, according to former colleagues and the owner of the stable where her horses were kept.

The day before the shooting, the school called her because Ethan had being searching online on his cell phone for information about ammunition during class.

She texted him, "lol I am not mad, you have to learn not to get caught."

The next day, the parents were summoned to the school after their son scribbled "my life is useless" and "blood everywhere," and drew a gun and a bloody body on the paper of his math quiz.

Jennifer Crumbley had shared a photo of the paper with her department head and with her riding instructor, saying she was just having "a shitty day".

But the couple left Ethan in school so they could go back to work. The teenager then took a gun out of his backpack and fired around thirty times indiscriminately.

The hearing will resume on February 23.