Nikolas Cruz, who has been charged with 17 counts of premeditated murder, reportedly left a comment on a YouTube video last year stating: “I’m going to be a professional school shooter.”
A user alerted authorities to the post.
After seeing a comment on a YouTube post last year by Mr Cruz, 19, user Ben Bennight contacted the FBI with his concerns. He said he spoke to representatives there for about 20 minutes.
Teachers were also warned about Mr Cruz, who was not allowed on campus with a backpack, US media report.
According to the BBC, the FBI was warned about the teenager who allegedly carried out a mass shooting at his former school in Florida.
Seventeen people were killed and many more were injured in the attack at Marjory Stoneman Douglas High School in Parkland, Florida. Several people are in a critical condition.
It is the deadliest shooting at a US school since 2012.
Mr Bennight said that the FBI had contacted him again following the school shooting in Parkland.
The FBI has not yet commented on the reports. (Excerpt from BBC News)