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.
