The FBI has launched a review over its handling of a warning about the teenager accused of carrying out Wednesday's school shooting in Florida.
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.