WINDER, Ga. (AP) — More than a year ago, tips about online posts threatening a school shooting led Georgia police to interview a 13-year-old boy, but investigators didn’t have enough evidence for an arrest. On Wednesday, that boy opened fire at his high school outside Atlanta and killed four people and wounded nine, officials said.

The teen has been charged as an adult in the deaths of Apalachee High School students Mason Schermerhorn and Christian Angulo, both 14, and 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, was 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.

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

But he returned later and wanted back in 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,” Sayarath said.

When she looked at him through a widow 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 math 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, 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 FBI narrowed the threats down and referred to the case to the sheriff’s department in Jackson County, which is adjacent to Barrow County.

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.

Hosey said the state Division of Family and Children’s Services also had previous contact with the teen and will investigate whether that has any connection with the shooting. 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 traumatized,” 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 in Barrow County, a rapidly suburbanizing area on the edge of metro Atlanta’s ever-expanding sprawl.

It was the the latest among dozens of school shootings across the U.S. 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.

Before Wednesday, there had been 29 mass killings in the U.S. so far this year, according to a database maintained by The Associated Press and USA Today in partnership with Northeastern University. At least 127 people have died in those killings, which are defined as incidents in which four or more people die within a 24-hour period, not including the killer — the same definition used by the FBI.

On Wednesday evening, hundreds gathered in Jug Tavern Park in downtown Winder for a vigil. Volunteers handed out candles and also water, pizza and tissues. Some knelt as a Methodist minister led the crowd in prayer after a Barrow County commissioner read a Jewish prayer of mourning.

Christopher Vasquez, 15, said he attended the vigil because he needed to feel grounded and be in a safe place.

He was in band practice when the lockdown order was issued. He said it felt like a regular drill as students lined up to hide in the band closet.

“Once we heard banging at the door and the SWAT (team) came to take us out, that’s when I knew that it was serious,” he said. “I just started shaking and crying.”

He finally settled down once he was at the football stadium. “I just was praying that everyone I love was safe,” he said.

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Associated Press journalists Sharon Johnson, Mike Stewart and Erik Verduzco in Winder; Beatrice Dupuy in New York; Russ Bynum in Savannah, Georgia; Charlotte Kramon, Kate Brumback and Jeff Martin in Atlanta; and Mark Thiessen in Anchorage, Alaska, contributed.