The 19-year-old man suspected of killing at least 17 people at a Florida high school had participated in paramilitary training with a white nationalist militia, the group's leader said on Thursday.
"He had some involvement with the Clearwater Republic of Florida cell at some point," said Jordan Jereb, the militia's leader, in a telephone interview about the suspected shooter.
The Federal Bureau of Investigation was warned last year about an ominous online comment by the alleged shooter but was unable to locate him, an agent said on Thursday.
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 US history.
He may also 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.
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.
Wednesday's shooting was the 18th in a US school this year, according to gun control group Everytown for Gun Safety. It stirred the long-simmering US debate on the right to bear arms, which are protected by the Second Amendment of the US 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.
Democrats in the US House of Representatives sharply criticized the Republican leadership for refusing to take up legislation on tightening background checks for prospective gun buyers.
"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 US senators, who were briefed 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.
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."
Bernie Woodall and Zachary Fagenson of Reuters reported from Parkland, Florida.