A 14-year-old gunman killed at least four people, including two students, and wounded nine more when he opened fire at a high school in Georgia on Wednesday, law enforcement said.
The shooter -- also a student at the school -- was taken into custody. He will be charged with murder and tried as an adult, the Georgia Bureau of Investigation said.
Two teachers were also among the dead.
After the latest chapter in America's gun violence crisis -- nearly 400 mass shootings this year alone, by one tally -- people gathered at a sports field outside Apalachee High School, some forming a circle with their arms linked.
"Our school resource officer engaged him," county sheriff Jud Smith told reporters, referring to law enforcement officers employed to work at US schools.
"The shooter quickly realized that if he did not give up that it would end with an OIS -- an officer-involved shooting. He gave up, got on the ground, and the deputy took him into custody."
Late Wednesday, the FBI revealed that the suspect had been brought to its attention more than a year ago for threats to commit a school shooting.
At that time the county sheriff's office interviewed the father and the then 13-year-old suspect, who denied the threats, before flagging the child to school officials for monitoring.
Smith said police did not yet know if the shooter singled out specific people as targets, and authorities have not yet identified the weapon.
Some in the school initially thought it was just another shooter drill, one student told AFP, referring to the exercises common in US schools.
"Everyone just thought it was a fake drill until my teacher said we didn't get an email," Alexsandra Romeo said.
"She got us all in a little corner and everyone was just hugging each other, I had some of my friends crying. Until two police officers came in with their guns and told us that this is not a drill and that we're still not safe."
Another student, 17-year-old Stephanie Folgar, described hearing "loud bangs" and panicking students hiding in the bathrooms and closets.
"It's scary knowing that that could've been you," she said.
One student told local media that he saw blood on the floor and a body as he was led out of the building by authorities.
The shooting occurred near the town of Winder, about 45 miles (70 kilometers) northeast of Atlanta, the state capital.
Earlier, school authorities were reported to have sent a message to parents saying they were enforcing a "hard lockdown after reports of gunfire."
After the all-clear was given, parents were invited to the school to be reunited with their children, with long lines of vehicles visible outside.
School shootings have become a sadly regular occurrence in the United States, where about a third of adults own a firearm and regulations on purchasing even powerful military-style rifles are lax.
Polls show a majority of voters favor stricter controls on the use and purchase of firearms, but the powerful gun ownership lobby is opposed to additional restrictions and lawmakers have repeatedly failed to act.
US President Joe Biden said he was mourning the dead.
"Students across the country are learning how to duck and cover instead of how to read and write. We cannot continue to accept this as normal," he said.
Speaking at a campaign event in New Hampshire after the shooting, Vice President Kamala Harris said it was time to end the "epidemic of gun violence."
Republican US presidential candidate Donald Trump said the perpetrator of the shooting was a "sick and deranged monster."
This year, there have been at least 384 mass shootings -- defined as a shooting involving at least four victims, dead or wounded -- across the United States, according to the Gun Violence Archive.
At least 11,557 people have been killed in firearms violence this year in the United States, according to the GVA.
