The FBI has said it did not properly follow up on a warning that Florida school shooting suspect Nikolas Cruz might carry out an attack.
The bureau's Public Access Line, which fields tips, did not pass along a complaint last month about the shooter.
"We have determined that these protocols were not followed," for the tip about Mr Cruz, said the bureau.
If the FBI Miami field office had received the complaint, action would have been taken, the statement reads.
Mr Cruz has confessed to carrying out the attack at Marjory Stoneman Douglas High School in Parkland and has been charged with 17 counts of murder.
It was the deadliest US school shooting since 2012.
On 5 January a person close to 19-year-old Cruz contacted the FBI tipline to provide "information about Cruz's gun ownership, desire to kill people, erratic behaviour, and disturbing social media posts, as well as the potential of him conducting a school shooting", said an FBI press release.
FBI Director Christopher Wray said the FBI is "still investigating the facts".
"I am 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," he said.