Prosecutor in the US state of Michigan announced Friday the parents of a suspected school shooter will each face four counts of involuntary manslaughter.

The four counts relate to the four students who were killed by shooting suspect 15-year-old Ethan Crumbley, on Tuesday afternoon, at Oxford High School, northwest of Detroit.

It is a very rare move for prosecutors to charge parents when their child is involved in a mass shooting, but Oakland County Prosecutor Karen McDonald said the behavior of Jennifer and James Crumbley was "so egregious, unconscionable. It is criminal."

Ethan Crumbley is charged with four counts of murder and seven counts of attempted murder, for the seven others who survived.

McDonald laid out the evidence against the parents: James Crumbley purchased the semiautomatic gun at a local gun shop with his son Ethan on Nov. 26, four days before the shooting, and Ethan later posted a photo of himself with the gun to social media with the words "Just got my new beauty today, any questions, I will answer." 15-year-olds are not allowed to possess guns under Michigan law.

Jennifer later posted a photo of herself and her son with the gun and the words "Mom and son day, testing out his new Christmas present".

Ethan was caught in class on Monday, Nov. 29, searching on his cell phone for ammunition. When a teacher called Jennifer Crumbley to let her know about the search, she did not reply, and instead she texted Ethan with the words: "LOL, I'm not mad at you. You have to learn not to get caught."

The following morning, the day of the shooting, McDonald said, a teacher saw a note on Ethan's desk with drawing of a person who appeared to be shot and bleeding, along with a laughing emoji and the words "The thoughts won't stop, help me, blood everywhere. My life is useless. The world is dead."

McDonald said Ethan's parents were immediately summoned to the school and were told they were required to get Ethan into counseling within 48 hours. But the parents resisted the idea of Ethan leaving the school at the time, and the parents did not search Ethan's backpack for the presence of the gun. He returned to the classroom, and minutes later, the shooting started.

McDonald said when news of the shooting broke out, Jennifer texted her son, "Ethan, don't do it."

McDonald also revealed that the gun was stored, unlocked, in Ethan's parents' bedroom, and that as the shooting unfolded, James Crumbley called police to report that the gun was missing and that he believed his son had it.

The charges against the parents, McDonald said, are designed to hold the parents accountable "and to send a message: that gun owners have a responsibility. When they fail to uphold that responsibility, there are serious consequences. We need to do better in this country. We need to say enough is enough."

McDonald would not say whether any school officials will face any criminal negligence charges for not doing more to prevent the shooting. Legal experts say at the moment, it appears the school and the school district will likely face civil lawsuits, not criminal charges, but McDonald said the criminal investigation is on-going.

She added that Michigan's gun laws are "woefully inadequate". Unlike some states, there is no requirement in Michigan that gun owners must keep their guns safely secured from children.

After the announcement of charges, national security and legal analyst Juliett Kayyem told CNN the behavior of the parents, as outlined by the prosecutor, was "shocking".

"As this kid is showing more and more evidence of really disturbing behavior, the parents ignore it, they 'LOL' the son, then they insist with the school that he gets to stay and don't look to see where the gun is."

Kayyem said the charges against the parents are "groundbreaking".

"But it feels absolutely right in my mind. I feel there is collective national jaws dropping. Good. It's about time parents are held responsible," added Kayyem./agencies