The Federal Bureau of Investigation (FBI) on Friday said it mishandled a January tip that the 19-year-old accused of killing 17 people in Florida had guns and the potential to carry out a school shooting.
A person close to accused gunman Nikolas Cruz called an FBI tip line on January 5 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 behaviour, and disturbing social media posts, as well as the potential of him conducting a school shooting," it said.
The tip appeared unrelated to a previously reported YouTube comment in which a person named Nikolas Cruz said, "I'm going to be a professional school shooter."
The FBI has acknowledged getting that tip as well but failing to connect it to Cruz, who is accused of carrying out the massacre at Marjory Stoneman Douglas High School in Parkland, Florida on Wednesday with an AR-15-style assault rifle.
"Under established protocols, the information provided by the caller should have been assessed as a potential threat to life," the FBI said in its statement. "The information then should have been forwarded to the FBI Miami field office, where appropriate investigative steps would have been taken. We have determined that these protocols were not followed."
The second-deadliest shooting at a public school in US history also raised concerns about potential failures in school security and stirred the ongoing US debate about gun rights, which are protected by the Second Amendment of the US Constitution.
"We are still investigating the facts," FBI Director Christopher Wray said in the statement. "We have spoken with victims and families, and deeply regret the additional pain this causes all those affected by this horrific tragedy."
Leaders including US President Donald Trump have linked mental illness to Wednesday's violence, suggesting that it was the public's responsibility to warn officials of such dangers.
"So many signs that the Florida shooter was mentally disturbed, even expelled from school for bad and erratic behavior," Trump said in a Thursday tweet. "Neighbors and classmates knew he was a big problem. Must always report such instances to authorities, again and again!"
Cruz, who had been expelled from the school where he allegedly staged his attack for undisclosed disciplinary reasons, made a brief court appearance on Thursday and was ordered held without bond.
"He's a broken human being," his lawyer, public defender Melissa McNeill, told reporters. "He's sad, he's mournful, he's remorseful."
Wednesday's shooting ranks as the greatest loss of life from school gun violence since the 2012 shooting rampage at Sandy Hook Elementary School in Newtown, Connecticut, that left 20 first-graders and six adult educators dead.
News of the FBI's mishandling of the last month's tip about Cruz came as families of the 17 victims began to bury their dead.
The first two funerals were for Alyssa Alhadeff, 14, a high school athlete and Meadow Pollack, an 18-year-old senior who had been headed to Lynn University in Boca Raton, Florida.
Brian Gately, a friend of the Alhadeff family, said he attended Alyssa's funeral and that the synagogue was so packed he had to stand in the rear.
"There was just really a lot of sadness in there," Gately, a 51-year-old financial adviser who lives in Parkland said. The burial became more emotional, he added, saying, "People were yelling, 'No, no.' Kids were yelling, 'No, no.'"
Trump tweeted on Friday morning that he would leave for Florida later in the day to meet people whose "lives had been totally shattered" by the shooting.
The vice mayor of Broward County, where the killings took place, blasted any visit by Trump, saying Republicans had failed to back common sense gun laws and had rolled back measures that made it harder for severely mentally ill people to buy weapons.
"Him coming here is absolutely absurd, and he's a hypocrite," Mark Bogen told CNN in an interview following Trump's tweet.