The Federal Bureau of Investigation was warned in September about an ominous online comment by the 19-year-old man accused of killing 17 people at his former high school but was unable to locate him, an agent says.
Ex-student, identified as Nikolas Cruz, walked into the Marjory Stoneman Douglas High School in Parkland, near Miami, on Wednesday and opened fire on students.
With 17 dead it is the second most deadly shooting in America's history - and the 18th school shooting at a US school this year.
Cruz may have left warning signs on social media in the form of a comment on a YouTube video that read "I'm going to be a professional school shooter." That comment was passed on to the FBI but they weren't able to track him down.
The FBI is conducting an extensive review of how it handled that tip to see if mistakes were made, a federal law enforcement official told Reuters.
It's also been revealed Cruz was also a white supremacist who had allegedly done paramilitary training with a white nationalist militia called the Republic of Florida, a leader of the group said.
"He had some involvement with the Clearwater Republic of Florida cell at some point," Jordan Jereb said in a telephone interview. Reuters could not immediately verify the claim.
Cruz loved guns and had been expelled from high school for disciplinary reasons, police and former classmates said.
He was also known for abusing animals with neighbours saying they were shocked by him shooting squirrels and rabbits on the neighbourhood, as well as his neighbour's chickens.
In a brief court appearance, Cruz spoke only two words, "Yes ma'am," when a judge asked him to confirm his name. He was ordered held without bond.
Cruz had recently moved in with another family after his mother's death in November, according to Jim Lewis, a lawyer representing the family and local media, bringing his AR-15 along with his other belongings.