The FBI has admitted that it failed to act on a tip-off about Florida school shooting suspect Nikolas Cruz last month.
17 people were killed in the Wednesday afternoon shooting at Marjory Stoneman Douglas High School in Parkland.
Cruz has confessed to carrying out the attack, according to a sheriff's department report.
The 19-year-old - who has been charged with 17 counts of premeditated murder - has been remanded in custody after appearing in court yesterday.
The suspect had previously been expelled from the school.
In a statement today, the FBI confirmed that a person close to Cruz had contacted the agency's public 'tip line' on January 5th to report concerns.
The caller is said to have expressed concern 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".
FBI protocols say that such information should have been considered a 'potential threat to life', and the details of the call forwarded to the FBI's Miami office.
However, the information was not sent to the Miami office, and no further investigation was conducted.
In the statement, FBI Director Christopher Wray said: "We are 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. It’s up to all Americans to be vigilant, and when members of the public contact us with concerns, we must act properly and quickly.
"We have spoken with victims and families, and deeply regret the additional pain this causes all those affected by this horrific tragedy."
According to details released by the local sheriff's office, the majority of the victims were teenagers aged between 14 and 18. Eight girls and six boys were killed.
There were also three adult victims - 37-year-old football coach Aaron Feis, 49-year-old athletic director Christopher Hixon, and 35-year-old geography teacher Scott Beigel.