Authorities said the alleged gunman, Nikolas Cruz, stopped at a WalMart, a Subway restaurant and a McDonald's immediately after carrying out the second-deadliest mass shooting at a public school in US history.
"The suspect crossed fields and ran west along with others who were fleeing and tried to mix in with the group that were running away, fearing for their lives," Broward Country Sheriff Scott Israel told reporters.
Cruz, who has been charged with 17 counts of premeditated murder, is accused of using a legally purchased AR-15-style weapon in Wednesday's attack, moving through Marjory Stoneman Douglas High School in Parkland outside Miami. Cruz was a former student of the school who had been expelled.
The accused shooter may have left warning signs on social media in the form of a comment on YouTube video that read "I'm going to be a professional school shooter." A YouTube user passed that comment on the Federal Bureau of Investigation in September, but the FBI did not connect it to Cruz.
"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. He noted that the YouTube user who reported the comment lives in Mississippi and said that investigators saw nothing to link it to Florida.
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 is 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."
Democrats in the US House of Representatives 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 reporters. "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.
Fifteen people were injured in Wednesday's shooting, according to local hospital officials.
'Broken Human Being'
"He's a broken human being," public defender Melisa McNeill told reporters. "He's sad, he's mournful, he's remorseful."
Cruz had 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.
Cruz told police that he had carried out the attack, according to an arrest document released on Thursday, which noted that he made that admission after being advised of his right not to incriminate himself.
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, 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, Lewis said.
Victims included an assistant football coach who sheltered students, a social science teacher and multiple students.
People who live on same street as Cruz said he alarmed them by shooting squirrels and rabbits in the neighbourhood as well as chickens being raised in a nearby backyard. Several times a year, they observed law enforcement officials at his house.
"Killing animals was no problem for this young man," said Rhonda Roxburgh, 45, who lived on the block for several years and whose parents continue to live there.
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