The FBI was tipped off about a YouTube comment from someone using the name “nikolas cruz” who said they were going to "be a professional school shooter” in September - five months before a man with same name allegedly opened fire on pupils in Florida.

The bureau had received information about the comment last year, said Robert Lasky, the special agent in charge of the Miami division.

But he said the tip did not include additional details that could help reveal the shape of a potential threat.

“No other information was included with that comment, which would indicate a time, location or the true identity of the person who made that comment,” he told reporters. “The FBI conducted database reviews, checks, but was unable to further identify the person who actually made the comment”.

In September, someone using the name "nikolas cruz" wrote: “Im going to be a professional school shooter”

It is unclear whether it is the same Nikolas Cruz, who has been charged with 17 counts of murder after the massacre at Marjory Stoneman Douglas High School in Parkland.

Authorities said 19-year-old saying Mr Cruz was a former student who had been expelled for bad behaviour.

Officials are now attempting to reconstruct Mr Cruz’s life in the lead up to the shooting and have been examining his online behaviour.

Broward County Sheriff Scott Israel said an initial review of websites and social media pages associated with Mr Cruz had yielded some “very disturbing” content.

He added that the shooter was wielding an AR-15, a type of semiautomatic rifle, and numerous magazines. A spokesman for the Department of Alcohol, Firearms and Tobacco later confirmed the gun was obtained legally.

Florida shooting – in pictures Show all 10 1 /10 Florida shooting – in pictures Florida shooting – in pictures Police arrest a suspect in connection with the shooting at the Marjory Stoneman Douglas High School in Parkland, Florida Reuters Florida shooting – in pictures Parents wait for news after reports of a shooting at Marjory Stoneman Douglas High School in Parkland, Florida AP Florida shooting – in pictures Anxious family members wait for news of students AP Photo/Wilfredo Lee Florida shooting – in pictures Students being evacuated from the school AP Florida shooting – in pictures Students being evacuated from the school Getty Florida shooting – in pictures People gather waiting for word from students AP Florida shooting – in pictures Parents waiting for news on their children AP Florida shooting – in pictures People gather at a hotel where students were taken after the shooting Getty Florida shooting – in pictures Florida Governor Rick Scott speaks to the media as he visits Marjory Stoneman Douglas High School following the shooting AFP/Getty Florida shooting – in pictures Dr. Igor Nichiporenko, Medical Director Trauma, left, and Dr. Evan Boyer, Medical Director, Emergency Services, speak about treating victims and the suspect at a press conference outside Broward Health North hospital AP

Students who knew Mr Cruz have described him as troubled. But Broward County Public Schools Superintendent Robert Runcie said school officials had not received any specific tips or seen red flags to alert them to danger.

“Potentially there could have been signs out there,” Mr Runcie told reporters outside of the school. “But we didn’t have any warning or phone calls or threats that were made”.

A maths teacher named Jim Gard who said Mr Cruz was one of his students has told reporters he recalled the high school’s administration emailing teachers about Mr Cruz.