Esteban Santiago, 26, was accused of killing five, wounding six and sending thousands scrambling for safety on Friday before authorities shut down the airport in Florida, a major gateway to the Caribbean and Latin America.
Federal prosecutors charged Santiago with firearms offenses and carrying out an act of violence at an airport, US Attorney Wifredo Ferrer said in a statement. If convicted, he could face the death penalty or life in prison.
The suspect was scheduled to make an initial court appearance on Monday.
Murder charges could be forthcoming from state prosecutors, but no decision has been made yet, a spokesman for Broward County State Attorney Mike Satz told the local Sun Sentinel newspaper.
Santiago had traveled from Alaska to Fort Lauderdale on Friday. After retrieving a 9mm semi-automatic handgun and ammunition that he had declared and stowed inside his checked luggage, he allegedly loaded the weapon in a bathroom and opened fire in the crowded baggage claim area of Terminal 2.
"Santiago started shooting, aiming at his victims' heads until he was out of ammunition," Ferrer said.
FBI special agent George Piro said law enforcement was continuing to investigate motives for the attack, including "continuing to look at the terrorism angle."
Piro said the suspect appeared to be acting alone and that "every indication is that he did follow (Transportation Security Administration) procedures in flying with the weapon."
The gunman was detained without law enforcement having to fire any shots, Broward County Sheriff Scott Israel said.
- 'Erratic' -
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.
An aunt, Maria Luisa Ruiz, told the NorthJersey.com news site that Santiago became a father to a baby boy in September -- and that he was having mental problems.
"Like a month ago, it was like he lost his mind," Ruiz said. "He said he saw things."
On November 7, Santiago walked into the FBI's Anchorage office and complained that his mind was being controlled by national intelligence agencies, which were forcing him to watch Islamic State jihadist videos, authorities said.
The "erratic behavior" led agents to contact local police, who took him to a medical facility for a mental health evaluation, Piro said.
He was not placed on a no-fly list.
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.
Santiago's weapon was taken by police for safekeeping at the time, and he was able to reclaim it on December 8.
Tolley said it was not known whether it was the same gun used in the rampage in Fort Lauderdale.
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 84-year-old grandmother Olga Woltering of Marietta, Georgia, the Sun Sentinel reported. The British native and her 90-year-old husband were going on a cruise with family to celebrate his birthday.
"Olga was so charming, calling everybody 'Lovey' or 'Love' in her unmistakable British accent," the Catholic Church of the Transfiguration, where Woltering was an active member, said on its website.
Terry Andres, 62, was going on vacation with his wife of nearly four decades, Ann, the Palm Beach Post reported. He lived in Virginia Beach, Virginia and worked at the Norfolk Naval Shipyard.
Michael and Kari Oehme, in their 50s from Council Bluffs, Iowa, were both shot, the Miami Herald said. Michael Oehme was killed, while his wife was wounded in the shoulder.
- Checking weapons -
The shooting renewed anxieties about airport security -- a concern that has loomed large in the post-9/11 era -- and shed new light on ongoing US gun-control debates.
The Transportation Security Administration -- the agency responsible for security at United States airports -- allows passengers to 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.
Speaking to CNN, she said the incident should prompt close 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.
"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."
Senator Chris Murphy, a Democrat from Connecticut, rebuked the government for not taking legislative action to tighten gun laws.
"Political cowardice is the accomplice of every mass shooter," he wrote on Twitter.
The Fort Lauderdale airport was open Saturday and staff were trying to return nearly 20,000 pieces of luggage and other personal items abandoned by passengers fleeing the shooting.