Florida shooting: Trump slams FBI over failure to probe tip on Nikolas Cruz

US President Donald Trump lashed out at the Federal Bureau of Investigation (FBI), describing its failure to follow up on a tip about the Florida school shooter Nikolas Cruz as "very sad", the media reported.

The FBI has said that it failed to act on information about the 19-year-old Cruz, who slaughtered 17 people on February 14 at the Marjory Stoneman Douglas school in Parkland, reports CNN.

The shooting reignited demands for tougher gun laws, with student survivors gathering in Fort Lauderdale earlier on Saturday to criticise Trump and other lawmakers for their inaction.

But Trump shifted blame to the FBI, saying it was devoting too much time to investigating his presidential campaign.

The FBI has acknowledged receiving two tips that appear to relate to Cruz ahead of the shooting. One was a January 5 call to a tip line from someone close to him - one that the FBI said it failed to act on, CNN reported.

The information should have been assessed as a "potential threat to life", but the proper protocols were not followed and the FBI's Miami office was not notified, the agency said.

Cruz, who has had a history of mental illness, is currently at the main Broward County jail facing 17 counts of premeditated murder.

His next court date is set for Monday.