Involuntary manslaughter charges have been filed against the parents of a 15-year-old accused of killing four students and wounding seven others at a Michigan school.

Ethan Crumbley is in custody after the attack on Tuesday at Oxford High School, about 30 miles north of Detroit.

Investigators say the handgun used was purchased legally by his father last week while Ethan was with him, and was “freely available” to the teenager.

“The parents were the only individuals in the position to know the access to weapons,” said Oakland County prosecutor Karen McDonald.

She said their actions went “far beyond negligence”.

James and Jennifer Crumbley have each been charged with four counts of involuntary manslaughter.

It is rare for people in the US to be charged over shootings by their children, even though most get the weapon from the house of their parents or another relative.

There is also no law in Michigan that says guns must be locked away from minors.

When he heard about the shooting, James Crumbley drove home to look for the gun and found it was missing, said Ms McDonald at a Friday news conference.

He then called authorities to say he suspected his son was responsible.

His parents had also met school officials about their son’s behaviour a few hours before the shooting, said Sheriff Mike Bouchard on Wednesday.

The teenager has been charged as an adult with crimes including murder, attempted murder and terrorism.

He is alleged to have emerged from a toilet and opened fire in a hallway. Seven students and a teacher were shot before he surrendered.

Three of the students died on Tuesday, and the fourth died on Wednesday.

Prosecutors said the teenager also made drawings and notes suggesting he was planning the attack, and the day before a teacher found a note with a drawing of a gun and blood.

The school was placed on lockdown after the attack, with children sheltering in locked classrooms while officers searched the premises.