WINDER, Ga. (AP) — Georgia police questioned a 13-year-old boy more than a year ago while investigating online posts threatening a school shooting. But investigators did not have enough evidence to make an arrest. On Wednesday, the boy opened fire on his high school outside Atlanta, killing four people and injuring nine, officials said.

The teenager was charged as an adult for using an assault rifle to kill two students and two teachers at Apalachee High School in the hallway outside his algebra classroom, Georgia Bureau of Investigation Director Chris Hosey said at a news conference.

It was Latest among the dozens of school shootings in the United States in recent years, including particularly deadly ones in Newtown, Connecticut, Parkland, FloridaAnd Uvalde, TexasThe classroom killings have sparked heated debates about gun control and frayed the nerves of parents whose children are growing up with classroom shooting drills. But little has changed in the nation’s gun laws.

When the teenager sneaked out of the classroom on Wednesday, Lyela Sayarath assumed her quiet classmate, who had recently transferred, was skipping school again. But he later returned and tried to go back into the classroom. Some students tried to open the locked door, but backed away instead.

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

The teenager then pointed the gun at people in a hallway.

He is accused of the deaths of students Mason Schermerhorn and Christian Angulo (both 14) and teachers Richard Aspinwall (39) and Christina Irimie (53), Hosey said. The teenager, now 14, was to be taken to a regional youth detention center on Thursday.

When the teenager was not allowed to return to his classroom, Sayarath said she heard a volley of gunshots.

“There were about 10 or 15 at a time, back to back,” she said.

The math students fell to the floor and crawled around, looking for a safe corner to hide in.

Two school security officers met the shooter minutes after receiving a report that shots had been fired, Hosey said. The teenager immediately turned himself in and was taken into custody.

At least nine other people – eight students and a teacher at the Winder school – were taken to hospitals with injuries. All are expected to survive, Barrow County Sheriff Jud Smith said. Authorities are still investigating how the teenager obtained the gun used in the shooting and brought it into the school of about 1,900 students in a rapidly suburbanizing area on the edge of the ever-growing Atlanta metropolitan area.

“All the students who had to watch their teachers and classmates die, those who had to leave school limping, looked traumatized,” said Sayarath.

It was the 30th. Mass killing in the US so far this year, according to a database maintained by The Associated Press and USA Today in cooperation with Northeastern University. At least 127 people have died in these killings, which are defined as incidents in which four or more people die within 24 hours, not including the killer – the same definition used by the FBI.

The teenager was questioned after the FBI received anonymous tips in May 2023 about online threats to carry out an unspecified school shooting, the agency said in a statement.

The FBI was able to isolate the threats and forward the case to the Jackson County Sheriff’s Department, which borders Barrow County.

The sheriff’s office interviewed the then 13-year-old and his father, who said there were hunting weapons 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 advised local schools that the teenager needed to be monitored, but there was no probable cause for an arrest or further action, the FBI said.

Hosey said the state Department of Family and Children Services also had previous contact with the teen and will investigate whether that has any connection to the shooting. Local news outlets reported that the teen’s family home in Bethlehem, Georgia, was searched Wednesday.

On Wednesday evening, hundreds gathered at Jug Tavern Park in downtown Winder for a vigil. Volunteers handed out candles. Some knelt while 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 felt grounded and in a safe place.

He was at band practice when the curfew was imposed. He said it felt like a normal practice as students lined up to hide in the band locker.

“When we heard a knock on the door and the SWAT team came to get us out, I knew it was serious,” he said. “I just started shaking and crying.”

Only when he reached the football stadium did he calm down. “I just prayed 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.