A teenage boy charged with killing four people at his high school in Georgia was interviewed last year by police about anonymous online threats, the FBI said.

The 14-year-old opened fire at his high school outside Atlanta, killing four people and wounding nine.

The teen has been charged as an adult in the deaths of Apalachee High School students Mason Schermerhorn and Christian Angulo, both 14, along with instructors Richard Aspinwall, 39, and Christina Irimie, 53, Georgia Bureau of Investigation director Chris Hosey said at a news conference.

At least nine other people — eight students and one teacher at the school in Winder, about an hour’s drive northeast of Atlanta — were taken to hospitals with injuries.

All were expected to survive, Barrow County sheriff Jud Smith said.

The teen, now 14, is to be taken to a regional youth detention facility on Thursday.

Armed with an assault-style rifle, the teen turned the gun on students in a hallway at the school when classmates refused to open the door for him to return to his algebra classroom, classmate Lyela Sayarath said.

The teen had earlier left the second period algebra classroom, and Miss Sayarath believed the quiet student who recently transferred was skipping school again.

But he returned later and wanted to get back into the classroom.

Some students went to open the locked door but instead backed away.

“I’m guessing they saw something, but for some reason they didn’t open the door,” Miss Sayarath said.

When she looked at him through a window in the door, she saw the student turn and heard a barrage of gunshots.

“It was about 10 or 15 of them at once, back-to-back,” she said.

The students ducked onto the floor and sporadically crawled around, looking for a safe corner to hide.

Two school resource officers encountered the shooter within minutes after a report of shots fired went out, Mr Hosey said.

The teen immediately surrendered and was taken into custody.

The teen had been interviewed after the FBI received anonymous tips in May 2023 about online threats to commit an unspecified school shooting, the agency said in a statement.

The sheriff’s office interviewed the then-13-year-old and his father, who said there were hunting guns in the house but the teen did not have unsupervised access to them.

The teen also denied making any online threats.

The sheriff’s office alerted local schools for continued monitoring of the teen, but there was no probable cause for arrest or additional action, the FBI said.

Local news outlets reported that law enforcement on Wednesday searched the teen’s family home in Bethlehem, Georgia, east of the high school.

“All the students that had to watch their teachers and their fellow classmates die, the ones that had to walk out of the school limping, that looked traumatised,” Miss Sayarath said.

“That’s the consequence of the action of not taking control.”

Authorities were still looking into how the teen obtained the gun used in the shooting and got it into the school with about 1,900 students.

It was the the latest among dozens of school shootings across the US in recent years, including especially deadly ones in Newtown, Connecticut, Parkland, Florida, and Uvalde, Texas.

The classroom killings have set off fervent debates about gun control and frayed the nerves of parents whose children are growing up accustomed to active shooter drills in classrooms.

But they have done little to move the needle on national gun laws.