Nikolas Cruz, 19, has appeared in court charged with premeditated murder. With 17 people killed, this is the deadliest US school shooting since 2012.
Mr Cruz reportedly commented on a YouTube post last year that he would be a “professional school shooter”.
The FBI admitted that a user had alerted authorities to the post.
Teachers were also warned about Mr Cruz, who was not allowed on campus with a backpack, US media report.
The FBI said it had investigated the YouTube comment but had been unable to fully identify the person who posted it.
The attack happened at Marjory Stoneman Douglas High School in Parkland, Florida. Fourteen people were wounded, three of whom remain in a critical condition.
On Thursday, Mr Cruz confessed to investigators that he carried out the shooting, adding that he had a backpack full of ammunition.
Expressing his condolences earlier, US President Donald Trump said he was “making plans to visit” Parkland.
After seeing a comment on a YouTube post last year by Mr Cruz, user Ben Bennight contacted the FBI and spoke to representatives for about 20 minutes.
Mr Bennight said the FBI contacted him again following the school shooting in Parkland.
The FBI confirmed on Thursday that they were made aware of the comment, adding that they had conducted “checks” but were unable to identify the person behind it.
Meanwhile maths teacher Jim Gard told the Miami Herald newspaper that school authorities had emailed teachers about Mr Cruz’s behaviour.
“We were told last year that he wasn’t allowed on campus with a backpack on him,” Mr Gard told the Miami Herald.
“There were problems with him last year threatening students, and I guess he was asked to leave campus.”
It is unclear why Mr Cruz was expelled from the school. Former schoolmate Joshua Charo said Mr Cruz had been found with bullets in his backpack.
“I can’t say I was shocked,” Mr Charo said after the shooting.
“He seemed like the kind of kid who would do something like this.”
Other students echoed that opinion. “Everyone predicted it,” one told local station WFOR.
But police said they had not been warned of a possible attack by Mr Cruz.
US media quote the head of white supremacist group the Republic of Florida Jordan Jereb as saying Mr Cruz had once trained with them, but the group had not wanted or ordered him to carry out a school shooting.
However local media later reported that investigators had been unable to find any link between Mr Cruz and the ROF. (Courtesy BBC)