The Justice Department on Saturday charged Esteban Santiago, 26, with firearms offences and carrying out an act of violence.
US authorities have charged the Iraq war vet accused of shooting five people dead and wounding six at a Florida airport with federal crimes that could carry the death penalty.
The Justice Department on Saturday charged Esteban Santiago, 26, with firearms offences and carrying out an act of violence when he opened fire at the busy Fort Lauderdale airport.
Santiago, who had earlier shown signs of "erratic behavior," arrived Friday on a flight from Alaska.
At baggage claim he retrieved a 9mm semi-automatic handgun and ammunition that he had declared and stowed inside his checked luggage, then allegedly loaded the weapon in a bathroom and opened fire in the crowded baggage claim area of Terminal 2.
Once his ammunition was exhausted Santiago lay on the floor with his arms and legs spread out and peacefully surrendered when a sheriff deputy approached him, witnesses quoted in US media said.
The hail of bullets sent thousands scrambling for safety and shut down the airport, a major gateway to the Caribbean and Latin America, for about 16 hours.
If convicted of the charges Santiago could face the death penalty or life in prison, US Attorney Wifredo Ferrer said in a statement.
"Santiago started shooting, aiming at his victims’ heads until he was out of ammunition," Ferrer said.
FBI special agent George Piro said agents were looking into the motives for the attack, including "continuing to look at the terrorism angle."
Piro said Santiago appeared to be acting alone and that "every indication" is that he followed rules in flying with the weapon.
Santiago is scheduled to make an initial court appearance on Monday.
A former member of the Puerto Rico and Alaska National Guard, Santiago served in Iraq from April 2010 to February 2011. He ended his service in August.
On November 7, Santiago walked into the FBI’s office in Anchorage, Alaska and complained that his mind was being controlled by national intelligence agencies, which were forcing him to watch Islamic State jihadist videos, authorities said.
This "erratic behavior" led agents to contact local police, who took him for a mental health evaluation, Piro said.
Anchorage police chief Christopher Tolley said Santiago came to the FBI office with a loaded magazine, but left his gun and newborn child in his car.
Police took Santiago’s weapon for safekeeping at the time, but he was able to reclaim it on December 8.
Tolley said it was unclear if that gun was used in the airport rampage.
Santiago’s brother, Bryan, criticized the way authorities handled his case.
"They had him hospitalized for four days and they let him go. How are you going to let someone leave a psychological center after four days when he said he hears voices that the CIA is telling him to join certain groups?" Bryan Santiago told CNN, in a Spanish-language interview the network translated into English.
"Not everyone has the same reaction when they return from war. Some are better, and some, not so much."
Authorities have not identified any victims, but three named in media reports were all getting ready to set off on cruises.
They included British-born 84-year-old grandmother Olga Woltering, who was going on a family cruise with her 90-year-old husband, the Sun Sentinel reported.
Terry Andres, 62, was going on vacation with his wife of nearly four decades, Ann, the Palm Beach Post reported.
Other victims included Michael and Kari Oehme, a couple in their 50s. Michael was killed while his wife was wounded in the shoulder, the Miami Herald reported.
The shooting renewed anxieties about security at US airports.
The Transportation Security Administration — the agency responsible for airport security — lets passengers travel with unloaded firearms and ammunition as checked baggage.
Congresswoman Debbie Wasserman Schultz, who represents the Florida district that includes the airport, said the rules need to be reexamined.
The incident should prompt a review of "whether or not you should be allowed to check a firearm at all" — as well as how passengers should be able to retrieve checked weapons after landing, she told CNN.
"If there is still exposure to the traveling public of someone who wishes to do them harm and, like this individual, was reunited with their firearm and was able to wreak havoc… it absolutely needs to be addressed."
Democratic Senator Chris Murphy rebuked the Republican-controlled Congress for not tightening gun laws.
"Political cowardice is the accomplice of every mass shooter," he wrote on Twitter. "The utter silence of Congress in the face of this carnage has become consent."
The Fort Lauderdale airport re-opened Saturday morning, and staff struggled to return nearly 20,000 pieces of luggage and other personal items abandoned by passengers who fled the shooting.