Authorities said the ex-student, identified as Nikolas Cruz, walked into the Marjory Stoneman Douglas High School in Parkland, near Miami, on Wednesday and opened fire with an AR-15-style assault rifle in the second-deadliest shooting at a public school in U.S. history.
Cruz may have left warning signs on social media. A person with his name wrote a comment last year under a YouTube video that read "I'm going to be a professional school shooter." The man who posted the video, Ben Bennight, a Mississippi bail bondsman, was alarmed and contacted the FBI, according to a video he posted online late Wednesday.
"No other information was included with that comment which would indicate a time location or the true identity of the person who made the comment," FBI Special Agent in Charge Robert Lasky told reporters on Thursday. Investigators were unable to find the commenter, he added.
Wednesday's shooting was the 18th in a U.S. school this year, according to gun control group Everytown for Gun Safety. It stirred the long-simmering U.S. debate on the right to bear arms, which are protected by the Second Amendment of the U.S. Constitution.
President Donald Trump addressed the shooting in a White House speech that emphasized school safety and mental health while avoiding any mention of gun policy.
"It is not enough to simply take actions that make us feel like we are making a difference," Trump said at the White House. "We must actually make that difference."
Broward County schools superintendent Robert Runcie called for action on gun laws.
"Now is the time for this country to have a real conversation on sensible gun control laws in this country," Runcie told a news conference.
"It's appalling," Representative Mike Thompson told a news conference. "Thirty people every day are killed by someone using a gun, and the best we can do is say we need more information?"
The Republican-controlled Congress last year revoked Obama-era regulations meant to make it harder for those with severe mental illness to pass FBI background checks for guns, saying the rule deprived the mentally ill of their gun rights.
At least one member of Trump's cabinet called for Congressional action.
"Personally I think the gun violence, it's a tragedy what we've seen yesterday, and I urge Congress to look at these issues," Treasury Secretary Steven Mnuchin told lawmakers on Thursday.
Authorities said Cruz wore a gas mask as he stalked into the school carrying a rifle, ammunition cartridges and smoke grenades. He then pulled a fire alarm, prompting students and staff to pour from classrooms into hallways, according to Florida's two U.S. senators, who were brief by federal authorities.
Cruz was expected to appear in court Thursday afternoon for a bond hearing, said Constance Simmons, a spokeswoman for the state attorney's office. He loved guns and was expelled for unspecified disciplinary reasons, police and former classmates said.
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.
The family believed Cruz was depressed, but attributed that to his mother's death, not mental illness.
"They didn't see any danger," Lewis told CNN.
South Florida was on edge on Thursday. Miami's main criminal courthouse building and another Broward school briefly went on lockdown due to security concerns, according to officials and local media. No injuries were reported in either incident.
The first victim of the attack was publicly identified on Thursday as Aaron Feis, an assistant coach on the school's football team and a school security guard who was shot while shielding students, the team said on Twitter.
An armed law enforcement officer stationed at the school did not encounter Cruz during his rampage, Broward County Sheriff Scott Israel told reporters.
Colton Haab, a 17-year-old member of the Junior Reserve Officer Training Corps at the high school, said he realized the alarms were not a drill after hearing several shots fired and helped barricade fellow students behind curtains made of Kevlar, a material used to make bullet-proof vests.
"We grabbed two pieces of two-by-four, a fire extinguisher and a chair," Haab said. "We were going to try to stop him with whatever we had."
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)