Investigators are trying to find out what made a 19-year-old student open fire at his former school in Florida, killing 17 people.
Police say they are beginning to “dissect” the websites and social media of suspect Nikolas Cruz, who was arrested after going on his shooting spree.
Broward County Sheriff Scott Israel described some of the things he had already become aware of as “very, very disturbing”.
Cruz is suspected of going into Marjory Stoneman Douglas High School in Parkland on Wednesday afternoon, armed with at least one AR-15 assault rifle, a gas mask, smoke grenades and several magazines of ammunition.
He deliberately set off the fire alarm so youngsters would leave their classrooms as he arrived at the scene, about 45 miles north of Miami, Florida Senator Bill Nelson told CNN.
He then targeted pupils.
Twelve people were shot dead in the school building, two outside, one in the street and two people were left with injuries from which they later died.
Among those who tried to save students was football coach Aaron Feis, who died after apparently using his body as a shield against the bullets.
The gunman was then detained without incident in the nearby town of Coral Springs after the Valentine’s Day massacre and taken to hospital with minor injuries.
It is not known when he is due to be interviewed.
Florida Senator Marco Rubio of Florida said in a statement he hoped authorities could find out in coming hours and days more about why and how the killer “carried out this carnage.”
Despite his former students saying there were warnings, authorities in Parkland appeared to claim there was nothing they could have done to prevent the attack.
Officials said Cruz was a former student at the school, but was expelled for unspecified disciplinary reasons.
Victoria Olvera, a 17-year-old at Marjory Stoneman Douglas, says Cruz was expelled last school year after a fight with his ex-girlfriend’s new boyfriend.
Another former friend, Dakota Mutchler, said he hadn’t seen him in more than a year after his expulsion before which “he started progressively getting a little more weird”.
Chad Williams, an 18-year-old at the high school, remembered Cruz as a troubled classmate from middle school.
He said Cruz would set off the fire alarm, day after day, and finally got expelled in the eighth grade.
More recently, Mr Williams saw Cruz carrying several publications about guns when they met by chance at the high school, where he thought Cruz was picking up a younger sibling.
“He was crazy about guns,” Mr Williams told Reuters. “He was kind of an outcast. He didn’t have many friends. He would do anything crazy for a laugh, but he was trouble.”
Maths teacher Jim Gard told the Miami Herald that Cruz had been forbidden from bringing a backpack on to the campus.
“There were problems with him last year threatening students, and I guess he was asked to leave campus,” Mr Gard told the newspaper.
Video has emerged that shows students cowering in a classroom as loud rapid shots are heard nearby.
McKenzie Hartley, 19, said her sister was inside the school at the time and she told Reuters: “She heard him shooting through the windows of classrooms and two students were shot.”
Broward County Mayor Beam Furr said told CNN Cruz had been getting treatment at a mental health clinic for a while, but had not been to the clinic for over a year.
“It wasn’t like there wasn’t concern for him,” Mr Furr said.
“We try to keep our eyes out on those kids who aren’t connected. Most teachers try to steer them toward some kind of connections. In this case, we didn’t find a way to connect with this kid,” he added.
Broward County School District Superintendent Robert Runcie told reporters he did not have any warning.
“Typically you see in these situations that there potentially could have been signs out there,” he said. “I would be speculating at this point if there were, but we didn’t have any warnings. There weren’t any phone calls or threats that we know of that were made.”
Sheriff 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.”
The injured were taken to two hospitals where at least three of them were critical and another three serious.
Authorities have embarked on a programme of counselling for anyone affected.
It is believed to be the second deadliest school shooting in US history, after Sandy Hook Elementary school in 2012 which left 28 dead.
Donald Trump tweeted: “My prayers and condolences to the families of the victims of the terrible Florida shooting. No child, teacher or anyone else should ever feel unsafe in an American school.”