The 14-year-old shooter who confessed to killing four people at Apalachee High School was allowed to leave his classroom with his belongings before returning with a rifle in his backpack, the Georgia Bureau of Investigation said Thursday.

The revelation supports previous student accounts that Gray had left his Algebra 1 class the morning of September 4 before carrying out the deadliest US school shooting this year in Winder, Georgia.

“Colt Gray brought the gun into the school on his own. The assault-style rife could not be broken down, but Gray hid it in his backpack,” the bureau said Thursday.

“Gray asked a teacher if he could go to the front office and speak to someone. The teacher allowed him to leave and take his belongings with him. Gray went to the restroom and hid from teachers. Later, he took out the rifle, and began shooting.”

Gray is charged with four counts of felony murder and will face additional charges, the Barrow County district attorney said. The 14-year-old will be tried as an adult; if convicted, he could face up to life in prison.

His father, Colin Gray, is charged with four counts of involuntary manslaughter, two counts of second-degree murder and eight counts of cruelty to children. According to an arrest warrant affidavit, Colin Gray gave his son a firearm “with knowledge he was a threat to himself and others.”

If convicted on all counts, the father could face a maximum prison sentence of up to 180 years.

The shooter and his father have not entered pleas. An attorney for Colt Gray declined to comment to CNN, and an attorney for Colin Gray has not responded to a request for comment.

Republican wants new gun safety measures

At least 389 mass shootings have taken place so far this year, according to the Gun Violence Archive, which defines mass shootings as those in which four or more victims are shot. That’s an average of more than 1.5 mass shootings every day.

In light of the Apalachee High School killings, Georgia House Speaker Jon Burns called for new gun safety and mental health measures in a letter to fellow Republicans.

“We are aware that a breakdown in communication between school systems and law enforcement officials allowed the shooter’s prior history to slip through the cracks upon his transfer to Apalachee High School,” Burns wrote in the letter Thursday to the Georgia House Republican Caucus.

“We must ensure our schools have the tools they need to coordinate resources for students who require mental health intervention.”

Investigation reveals details about the school shooter’s past and access to firearms

The house speaker promised to reintroduce legislation that would incentivize safe gun storage and encourage gun safety training. Burns also proposed looking for new technologies to detect firearms outside school buildings and an expansion of access to mental health care across the state.

Burns’ letter came a day after Democratic state leaders, including Senate Minority Leader Gloria Butler, joined gun violence survivors at the Georgia State Capitol on Wednesday demanding gun safety legislation, including safe storage measures and red-flag laws.

This story has been updated with additional information.

CNN’s Jamiel Lynch contributed to this report.