Someone close to Nikolas Cruz called the FBI’s public tip-off line on 5 January and gave information about his guns and erratic behaviour, including his expressed desire to kill people and his disturbing social media posts.
The caller contacted the law enforcement agency to warn of “the potential of him conducting a school shooting”.
The FBI admitted that the tip-off should have been shared with its Miami office and investigated, but it was not.
FBI director Christopher Wray said the agency was reviewing what went wrong.
Cruz, 19, is accused of using a semi-automatic rifle to kill 17 people at Marjory Stoneman Douglas High School in Parkland, north of Miami, in one of the nation’s deadliest school shootings. He carried extra ammunition in his backpack, according to a sheriff’s department report.
Cruz told investigators he shot students in the hallways and on the grounds of the school.
As he moved through the school, he fired into five classrooms: four on the first floor and one on the second floor, Sheriff Scott Israel said.
The shooting lasted for three minutes. The assailant then went to the third floor and dropped his AR-15 rifle and the backpack and ran out of the building, blending in with fleeing students, Israel said.
He then headed to a Wal-Mart and bought a drink at a Subway restaurant before walking to a McDonald’s. He was taken into custody about 40 minutes after leaving the burger bar, the sheriff said.
A day after the attack, a fuller portrait emerged of the shooter, a loner who had worked at a dollar store, joined the school’s Reserve Officers’ Training Corps programme and posted photographs of weapons on Instagram.
At least one pupil said classmates joked that Cruz would “be the one to shoot up the school”.
The 19-year-old orphan whose mother died last year was charged with murder on Thursday. It was the nation’s deadliest school attack since a gunman targeted Sandy Hook elementary school in Newtown, Connecticut, more than five years ago.
Pupils struggled to describe the violence that ripped through their classrooms just before the school day ended.
Catarina Linden, 16, said she was in a maths class when the gunfire began.
“He shot the girl next to me,” she said, adding that when she was able to leave the classroom, the air was foggy with gun smoke.
“I stepped on so many shell casings. There were bodies on the ground and there was blood everywhere.”
Among the dead was Aaron Feis, an assistant American football coach and security guard. He is being hailed as a hero for shielding students from bullets. He was fatally injured after diving in front of a pupil.
Also killed were a senior who planned to attend Lynn University and an athletics director who was active in his Roman Catholic church.
The last of the bodies were removed from the school on Thursday after authorities analysed the crime scene. Thirteen wounded survivors were still in hospital, including two in critical condition.
Authorities have not offered any motive, except to say that Cruz had been expelled from the school, which has about 3,000 pupils and serves an affluent suburb on the edge of the Everglades.
Pupils who knew him described a volatile teenager.
President Donald Trump tweeted yesterday that he plans to meet relatives of the victims, many of whom attended a vigil for those killed.