Nikolas Cruz, 19, is in custody accused of massacring 17 people, including students and teachers, at a high school in Florida.
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 multiple rounds of ammunition.
He raised the fire alarm in order to have people leave classrooms and spill out into the corridor to give him the opportunity to carry out his deadly attack.
Details have since started to emerge about Cruz, as police reveal details of their investigation and fellow students share stories about their encounters with him.
:: He was expelled from school
Victoria Olvera, a 17-year-old student, said Cruz was expelled last year because he got into a fight with his ex-girlfriend's new boyfriend. She said he had been abusive to his girlfriend.
A 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".
:: People ended friendships with him
Students who knew him described a volatile teenager whose strange behaviour had caused others to end friendships with him.
:: "Very disturbing things have been found"
Police say they are beginning to "dissect" Cruz's internet history and social media.
Broward County Sheriff Scott Israel described some things they had already found as "very, very disturbing".
Soon after the arrest of the suspect, YouTuber Ben Bennight recounted an apparent run-in with Cruz on his account.
The vlogger said an account in the same name as the suspect left a comment on one of his videos several months ago saying he wanted to be a "professional school shooter".
Mr Bennight, who runs the account Benthebondsman, reported the comment to the FBI, who interviewed him about it. He said he received a call from FBI agents after Cruz's arrest, but the agency has not confirmed this.
The FBI has not confirmed the account was linked to Cruz.
:: He was "troubled"
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.
:: His adoptive mother died in November
Cruz and his biological brother Zachary were adopted by Lynda Cruz and her husband when they moved from Long Island to Broward County.
His adoptive father, Roger, died of a heart attack several years ago. His mother died of pneumonia in November last year, aged 68.
He went to live with a family friend, but was unhappy there and moved in with a friend and his family in northwest Broward.
They knew he had a gun, and made him keep it in a locked cupboard. He was able to keep the key.
The family he was staying with have not been named. They are said to be "devastated" by what happened.
:: "He was crazy about guns"
Chad saw Cruz carrying several publications about guns when they met by chance at the high school recently. He thought Cruz was there to pick up a younger brother or sister.
"He was crazy about guns… he was kind of an outcast. He didn't have many friends. He would do anything crazy for a laugh, but he was trouble.
"I think everyone had in their minds if anybody was going to do it, it was going to be him," Victoria Olvera said.
:: He was banned from bringing his backpack into school
"There were problems with him last year threatening students, and I guess he was asked to leave campus," Maths teacher Jim Gard told the Miami Herald.
:: He was part of the US military-sponsored Junior Reserve Officers' Training Corp
Jillian Davis, 19, says Cruz was part of the programme as a high school freshman.
Davis is a graduate and former fellow JROTC member.
::He belonged to a white nationalist group
The leader of a group called Clearwater Republic of Florida said Cruz belonged to one of their cells and took part in some of their militia's activities involving the use of weapons.
The group wants Florida to become its own white ethno-state. Jordan Jereb said Cruz acted "solely on his own behalf".
:: He was calm during his arrest
Cruz was taken into custody without a fight about an hour after the shooting.
More from Florida
Michael Nembhard was sitting in his garage on a cul-de-sac when he saw a young man in a burgundy shirt walking down the street. In an instant, a police cruiser pulled up, and officers jumped out with guns drawn.
"All I heard was 'Get on the ground! Get on the ground!'" Nembhard said. He said Cruz did as he was told.