"On January 5, 2018, a person close to Nikolas Cruz contacted the FBI's Public Access Line tipline to report concerns about him," the FBI said in a statement.
"The caller provided information about Cruz's gun ownership, desire to kill people, erratic behavior, and disturbing social media posts, as well as the potential of him conducting a school shooting," it said.
The information was not handled appropriately, however, the top US law enforcement agency said, and no action was taken.
Cruz, 19, shot dead 17 people at a high school in Parkland, Florida, on Wednesday using an AR-15-style rifle that he had legally purchased.
Cruz, who had a history of troubling behavior, was expelled from the school last year for "disciplinary reasons."
This was not done and "no further investigation was conducted at that time," it said.
FBI Director Christopher Wray said he was "committed to getting to the bottom of what happened in this particular matter, as well as reviewing our processes for responding to information that we receive from the public.
"We have spoken with victims and families, and deeply regret the additional pain this causes all those affected by this horrific tragedy," Wray said.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)