ATLANTA – The 14-year-old boy accused of fatally shooting four people and wounding nine others at his Georgia high school was allowed to leave his classroom with his belongings before he returned and opened fire with an AR-15-style rifle that he had smuggled into the school in his backpack, investigators said Thursday.

The Georgia Bureau of Investigation confirmed witness accounts from classmates who have said Colt Gray had left his second-period classroom ahead of the deadly Sept. 4 shooting at Apalachee High School in Winder, Ga., about 35 miles northeast of Atlanta, and returned with a gun.

“Gray asked a teacher if he could go to the front office and speak to someone,” GBI said in a statement. “The teacher allowed him to leave and take his belongings with him.”

But investigators said Gray instead “went to the restroom and hid from teachers” before emerging to begin the attack. GBI said Gray brought the gun used in the shooting into the school on his own.

“The assault-style rifle could not be broken down, but Gray hid it in his backpack,” the statement said.

Alfonso Kraft Jr., a public defender who is representing Colt Gray, through a spokesman declined to comment on the GBI statement.

The fresh details came as attorneys for Colin Gray, the suspected shooter’s father who is also facing charges in the attack, filed a motion in Barrow County Superior Court seeking to have their client isolated from other inmates at the local jail where he is being held without bond. They cited death threats that the elder Gray had received since last week’s shooting.

Intense news coverage and social media discussion of the case had led to an “incalculable number of threats” against Colin Gray, “calling for both harm and violence … and in some cases even the death of the defendant,” his attorneys, Jimmy D. Berry and Brian A. Hobbs, wrote in a motion filed in Superior Court.

Berry and Hobbs, who joined the case on Wednesday, did not detail any specific threats from other inmates at the Barrow County Detention Center. But they argued that it would be “reckless to assume” there are no inmates now or in the future who might not wish harm against Colin Gray because of “the feelings of anger and retribution manifested in the collective psyche of both the public and the community at large.”

As of Thursday, a judge had not been formally named to preside over the pending cases against Colt and Colin Gray, and it was not immediately clear when or if the court would take up Colin Gray’s motion.

Colt Gray is charged as an adult with four counts of felony murder, with more charges expected in the coming weeks, according to Barrow County District Attorney Brad Smith. He faces life in prison if convicted.

Authorities arrested and charged Colin Gray, 54, two days after the deadly shooting, accusing him of “providing a firearm” to his son even as he had “knowledge that he was a threat to himself and others.” The elder Gray is facing two counts of second-degree murder, four counts of involuntary manslaughter and eight counts of cruelty to children and faces upward of 180 years in prison if convicted.

Neither Gray has entered a formal plea in the case.