At least 17 people were dead after a 19-year-old man opened fire at Marjory Stoneman Douglas High School in Parkland, Florida, on Wednesday afternoon.
Fourteen others were wounded, five of whom had life-threatening injuries Wednesday night.
Authorities said the suspected gunman, identified as Nikolas Cruz, concealed himself in the fleeing crowd and was arrested in nearby Coral Springs.
He was treated for “labored breathing” as a precaution but was released from the hospital, authorities said.
Little was known about Cruz on Wednesday night. Broward County schools Superintendent Robert Runcie said he had recently been expelled from Douglas for disciplinary reasons and was currently enrolled in another school in the district.
General store chain Dollar Tree confirmed that Cruz worked at its Parkland store and said it was fully cooperating with investigators.
Broward County Sheriff Scott Israel, who told reporters Wednesday night that the son of one of his deputies was among the injured, said Cruz was believed to have been armed with an AR-15-style semiautomatic rifle and multiple magazines. Israel said it was unclear whether he had any other weapons.
The gunfire began outside the school and continued inside, where 12 of the victims were killed.
All of those victims have been identified but no identities will be made public until the families of all of those affected have been notified.
Federal and local authorities told NBC News that there was no indication that the gunman had an accomplice or accomplices.
Cruz was taken into custody off campus about an hour after he “committed this horrific, detestable act,” said Israel, who said investigators were reviewing social media postings that he described as “very disturbing.”
Eddie Bonilla, a senior, told NBC Miami that Cruz was “a little bit off” and “troubled.”
Cruz used to show off his guns, brag about shooting them “for fun” and “threatened to bring the guns to school multiple times,” he said, adding that students “threw jokes around that he’d be the one to shoot up the school.”