Nikolas Cruz has been charged with 17 counts of premeditated murder, after a shooting inside his former high school in Broward County.
He was armed with an AR-15 assault rifle, several rounds of ammunition, a gas mask and smoke grenades.
It has now emerged that his strange behaviour may have been known to the FBI in the months before the mass shooting on Valentine's Day.
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YouTuber Ben Bennight, who runs an account called Benthebondsman, said he received a comment on one of his videos from a user called Nikolas Cruz.
The comment read: "I'm going to be a professional school shooter."
Mr Bennight flagged the comment to YouTube after taking a screenshot, and it was removed, but he felt he should do more and managed to get in touch with the FBI.
He said he was interviewed by FBI agents soon after he made the report, but did not hear anything again.
Mr Bennight then received another phone call within 30 minutes of Cruz's arrest on Wednesday.
Former classmates of Cruz also suggest his behaviour was "strange", and recounted that he was expelled because he got into a fight with his ex-girlfriend's new boyfriend.
Chad Williams said he spotted Cruz at the high school in a chance meeting, and realised he was carrying several publications about guns.
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.
He was also receiving treatment at a mental health clinic, but had not been for an appointment in a year.
Broward County Mayor Beam Furr told CNN: "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."
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."
Despite a warning to the FBI, and his previous behaviour in school, authorities have said there was nothing they could have done to prevent the attack.
Cruz and his brother Zachary were adopted by an older couple, Lynda and Roger Cruz.
The brother lost their adoptive father several years ago to a heart attack, and Mrs Cruz died in November last year.
They lived with a family friend, but Cruz was unhappy, and asked to move out. He then lived with his friend's family in a different part of Broward County.
The family the suspect was living with said they were devastated by the shooting and "did not see it coming".
James Lewis, a lawyer for the family, which has not been named, said: "[He] didn't have any other place to go and they had an extra room.
"And he lived here, lived among them. And was going to school at an adult education centre, apparently was working at a dollar store.
"[He] seemed to be getting his life together, was getting over the depression, the loss of his mother. This totally came out of left field. They had no idea, no predilection."
The family said through the lawyer that they were "unaware" of any strange behaviour on social media.
The night before the shooting, Cruz went to work, and on the morning of the shooting, told his friend he "didn't go to school on Valentine's Day".
Mr Lewis said: "He didn't think anything of it at the time and I really can't place it why, but the only thing strange, that's the only strange comment. The only strange thing that happened today.
"So why this happened? Nobody saw it coming, nobody knows why."
The massacre killed 17 people, including football coach Aaron Feis who used his body as a shield for students while shots raged.
Jaime Guttenberg was also confirmed dead by her family on Thursday, as her cousin left an emotional tribute.
As well as 17 dead, there were 14 wounded survivors taken to hospital.
It is thought Cruz set off the fire alarm in the school to draw more people into the corridors before opening fire.