Ethan Crumbley, accused of killing four students at a Michigan school on Tuesday has been charged as an adult with two dozen crimes, including murder, attempted murder, and terrorism.

Four students were killed at Oxford High School in Oakland County, roughly 30 miles (50 kilometers) north of Detroit. Seven more people were injured, including a student who remained in critical condition.

A prosecutor considering criminal charges against the parents of the 15-year-old boy said their actions went “far beyond negligence” and that the gun “seems to have been just freely available” to the teenager.

“All I can say at this point is those actions on mom and dad’s behalf go far beyond negligence,” Oakland County prosecutor Karen McDonald said. “We are prosecuting the shooter to the fullest extent. … There are other individuals who should be held accountable.”

The semi-automatic gun was purchased by Crumbley’s father last week, according to investigators.

“The parents were the only individuals in the position to know the access to weapons,” McDonald told WJR-AM. The gun “seems to have been just freely available to that individual.”

No Michigan law requires gun owners to keep weapons locked away from children.

William Swor, a defense lawyer who is not involved in the case, said charging the parents would require a “very fact-intensive investigation.”

“What did they know and when did they know it?” Swor said. “What advance information did they have about all these things? Did they know anything about his attitude, things of that nature? You’re talking about a very heavy burden to bring on the parents.”

On Thursday, dozens of schools in southeastern Michigan canceled classes due to concerns about threatening messages on social media following the Oxford shooting.

Some schools stayed open with a larger police presence.