FORT LAUDERDALE, Fla. -- The Iraq War veteran accused of killing five travelers and wounding six others at a busy international airport in Florida was charged Saturday and could face the death penalty if convicted.

Esteban Santiago, 26, told investigators he planned the attack, buying a one-way ticket to the Fort Lauderdale airport, a federal complaint said.

Authorities don't know why he chose his target and have not ruled out terrorism.

Santiago was charged with an act of violence at an international airport resulting in death -- which carries a maximum punishment of execution -- and weapons charges.

"Today's charges represent the gravity of the situation and reflect the commitment of federal, state and local law enforcement personnel to continually protect the community and prosecute those who target our residents and visitors," federal prosecutor Wifredo Ferrer said.

Law-enforcement personnel tell people to take cover Friday at Hollywood International Airport in Fort Lauderdale, Florida. A gunman opened fire in the baggage claim area at the airport Friday, killing several people and wounding others before being taken into custody in an attack that sent panicked passengers running out of the terminal and onto the tarmac, authorities said.Wilfredo Lee ~ Associated Press

Authorities said during a news conference they had interviewed about 175 people, including a lengthy interrogation with the cooperative suspect, a former National Guard soldier from Alaska.

Flights had resumed at the Fort Lauderdale airport after the bloodshed, though the terminal where the shooting happened remained closed.

Santiago spoke to investigators for several hours after he opened fire with a Walther 9 mm semi-automatic handgun he appears to have legally checked on a flight from Alaska. He had two magazines with him and emptied both of them, firing about 15 rounds, before he was arrested, the complaint said.

"We have not identified any triggers that would have caused this attack. We're pursuing all angles on what prompted him to carry out this horrific attack," FBI agent George Piro said.

Investigators are combing through social media and other information to determine Santiago's motive, and it's too early to say whether terrorism played a role, Piro said.

In November, Santiago had walked into an FBI field office in Alaska saying the U.S. government was controlling his mind and forcing him to watch Islamic State group videos, authorities said.

"He was a walk-in complaint. This is something that happens at FBI offices around the country every day," FBI agent Marlin Ritzman said.

On that day, Santiago had a loaded magazine on him but had left a gun in his vehicle, along with his newborn child, authorities said.

Officers seized the weapon and local officers took him to get a mental-health evaluation. His girlfriend picked up the child.

On Dec. 8, the gun was returned to Santiago. Authorities wouldn't say whether it was the same gun used in the airport attack.

Santiago had not been placed on the U.S. no-fly list and appears to have acted alone, authorities said.

The attack sent panicked witnesses running out of the terminal and spilling onto the tarmac, baggage in hand. Others hid in bathroom stalls or crouched behind cars or anything else they could find as police and paramedics rushed in to help the wounded and establish whether there were any other gunmen.

Mark Lea, 53, had just flown in from Minnesota with his wife for a cruise when he heard three quick cracks, like a firecracker. Then came more cracks, and "I knew it was more than just a firecracker," he said.

Making sure his wife was outside, Lea helped evacuate some older women who had fallen, he said. Then he saw the shooter.

"He was just kind of randomly shooting people," he said. "If you were in his path, you were going to get shot. He was walking and shooting."

Over the course of about 45 seconds, the shooter reloaded twice, he said. When he was out of bullets, he walked away, dropped the gun and lay face down, spread eagle on the floor, Lea said.

By that time, a deputy had arrived and grabbed the shooter. Lea put his foot on the gun to secure it.

Lea went to help the injured and a woman from Iowa asked about her husband, who she described. Lea saw a man who fit his description behind a row of chairs, motionless, shot in the head and lying in a pool of blood, he said. The man, Michael Oehme, was identified as one of the dead victims Saturday.

Santiago will make his first court appearance Monday.

It is legal for airline passengers to travel with guns and ammunition if the firearms are put in a checked bag -- not a carry-on -- and are unloaded and locked in a hard-sided container. Guns must be declared to the airline at check-in.

Despite his mental evaluation, federal prosecutor Karen Loeffler said Santiago would have been able to legally possess a gun because he had not been judged mentally ill, which is a high standard.

Santiago arrived in Fort Lauderdale after taking off from Anchorage aboard a Delta flight Thursday night, checking only one piece of luggage -- his gun.