Authorities in Florida could offer no explanation Wednesday night as to why a former student armed with an AR-15 rifle opened fire at a high school earlier that day, killing at least 17 people.
Broward County Sheriff Scott Israel identified the gunman as Nikolas Cruz, 19, a former student at Marjory Stoneman Douglas High School in Parkland who had been expelled for "disciplinary reasons," but was currently enrolled in Broward County Public Schools.
Cruz, whose fellow students described him as "troubled," was arrested without incident in the nearby town of Coral Springs after the Valentine's Day rampage and taken to hospital with minor injuries, the sheriff said.
He had mixed in with students fleeing the school before being caught. Cruz has been charged with 17 counts of premeditated murder, officials said.
"We have already begun to dissect his websites and things on social media that he was on and some of the things... are very, very disturbing," Israel said.
"If a person is predisposed to commit such a horrific event by going to a school and shooting people ... there's not anybody or not a lot law enforcement can do about it."
Israel said both students and adults had been killed, 12 of whom have now been identified.
Fifteen people were killed at the school itself and 17 were taken to hospital, two of whom died of their wounds, the sheriff's office said. One of those killed was a football coach, and one student injured was a deputy sheriff's son.
"This is a terrible day for Parkland," Israel said, speaking of the city of about 30,000 people, located 50 miles (80 kilometres) north of Miami.
A teacher at the school said Cruz had been identified previously as a potential threat to his classmates.
"We were told last year that he wasn't allowed on campus with a backpack on him," math teacher Jim Gard said in a Miami Herald interview.
"There were problems with him last year threatening students, and I guess he was asked to leave campus."
Cruz was also said to have been in the Junior ROTC (Reserve Officers' Training Corps) program while at school.
A law enforcement source told CBS News that the gunman pulled a fire alarm before opening fire, but Israel could not confirm that report.
EVERYONE STARTED RUNNING
The shooting, one of nearly 20 at a school since the start of the year, will once again throw the spotlight on the epidemic of gun violence in the United States, where there are 33,000 gun-related deaths annually.
But when questioned at a press conference late Wednesday, Florida Governor Rick Scott -- who described the massacre as "just pure evil" -- declined to make a statement on gun control in the aftermath of the shooting.
"There's a time to continue to have these conversations about how through law enforcement, how through mental illness funding that we make sure people are safe, and we'll continue to do that," said Scott, a Republican.
At the same briefing, Florida Attorney General Pam Bondi said the state will cover the costs of funerals and counselling for survivors.
"We will continue to work together as a team, as a family, and love and take care of all of these victims and their family members," she said.
Parkland Mayor Christine Hunschofsky told CNN she had spoken to a number of students after the shooting erupted shortly after 2:00 pm (1900 GMT).
"They were very scared," she said. "And almost in shock when they came out."
Police officers in helmets, bulletproof vests and armed with automatic weapons could be seen stationed at several points around the sprawling school complex, which serves nearly 3,000 students.
The FBI said it was assisting local law enforcement with the investigation.
US President Donald Trump yesterday urged Americans to report neighbours' "erratic" behaviour as he faced down calls for action after 17 people were shot dead at a Florida school.
"So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior," Trump tweeted.
"Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!"
He ordered flags at US embassies, government buildings and military installations to be flown at half staff yesterday, after the shooting incident.
In a proclamation, Trump ordered the measure "as a mark of solemn respect for the victims of the terrible act of violence perpetrated on February 14, 2018."
"Our nation grieves with those who have lost loved ones in the shooting at the Marjory Stoneman Douglas High School in Parkland, Florida," he said.