The Iraq war veteran accused of killing five airline passengers and wounding six others in Florida has been charged with an act of violence at an international airport resulting in death – an offence that carries a maximum punishment of execution if convicted.

Esteban Santiago, 26, also faces weapons charges over the attack at Fort Lauderdale international airport. A federal complaint said he admitted to planning the attack and had bought a one-way ticket to the airport.

“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,” said US attorney Wifredo Ferrer.

Authorities said they had interviewed roughly 175 people, including a lengthy interrogation with the co-operative suspect, a former National Guard soldier from Alaska. Flights had resumed at the Fort Lauderdale airport, though the terminal where the shooting happened was still affected.

On Saturday the FBI agent in charge of investigating the case said terrorism was being considered as a possible motive.

“We continue to look at all angles and motives and at this point we are continuing to look at the terrorism angle,” said George Piro, a special agent in charge of the FBI’s Miami bureau.

“The indications are he came here to carry out this horrific attack. We have not identified any triggers that would have caused this attack but we have not ruled anything out.”

Piro said agents had concluded an “hours-long” interview with Santiago early on Saturday but refused to go into details, citing the ongoing investigation.

Santiago is expected to make his first court appearance on Sunday. He is a National Guard veteran who, his family said, developed psychological problems after returning in 2011 from a tour of duty in Iraq. He made no attempt to resist when he was arrested in the baggage hall of the airport’s Terminal 2 on Friday.

Investigators said he had flown to Fort Lauderdale from his home in Alaska, with a stopover in Minneapolis, and appeared to have acted alone. Piro said the suspect used a legally held 9mm semi-automatic handgun, which had been checked on to the flight in accordance with security requirements.

At an earlier briefing Piro said Santiago, who was discharged from the Alaska National Guard for “unsatisfactory service” in August, had turned up at the FBI’s office in Anchorage in November complaining that voices in his head were telling him to follow Isis.

Santiago’s aunt, Maria Ruiz Rivera, meanwhile, said her nephew “had visions all the time”. Speaking to reporters from her apartment in New Jersey, she said he had become increasingly unstable after returning from Iraq. “His mind was not right. He seemed normal at times but other times he seemed lost. He changed.”

Several witnesses told how they escaped the attack, including a man who said his life was saved by a laptop in his backpack. Steve Frappier, from Atlanta, Georgia, said he was trying to shelter on the floor of the baggage hall “like a tortoise with the backpack on me” when he felt something hit him. “The bullet entered my backpack [and] hit my laptop,” he told CNN.

A woman from Weston, Florida, who asked not to be named, said the gunman walked around the baggage carousel while shooting. “He was just walking with his arms straight out, stony-faced,” she said, adding that a woman who was standing next to her was killed by a shot to the head.

Another witness, Mark Lea, from Minnesota, spoke of helping those who had been shot, including a woman with a shoulder wound who was looking for her husband.

“I saw that she had a through-and-through on the right shoulder, and she said, ‘where’s my husband, where’s my husband?’” Lea told KETV. “I asked her to describe him and I looked right over there and saw a white-haired guy in a blue shirt. He was not moving, not breathing.”

Family members named an elderly couple from Council Bluffs, Iowa, as victims, saying the husband had died and that his wife was recovering in hospital from gunshot wounds to her neck and shoulder. Media reports also identified a 62-year-old grandfather from Virginia as among the dead, as well as a great-grandmother from Georgia who was in Fort Lauderdale for a cruise to celebrate her husband’s 90th birthday.

Authorities were yet to formally identify the victims, but the number of wounded was reduced from eight to six. Three were described as being in good condition, while three remained in intensive care.

On Saturday morning Rick Scott, Florida’s governor, said he had visited some of the victims of the “absolutely horrific day” at Broward Health Medical Center. “We all want answers. Individuals have been killed and some are fighting for their lives,” he said.

“I’m a dad and I’m a granddad, I just can’t imagine this happening to my family or any other family.”

Scott repeated a promise that the killer would be held responsible “to the fullest extent of the law” and tried to reassure tourists that Florida was safe to visit.

“We love our tourists and we’ll do everything we can to encourage them to come here,” he said, claiming that crime in the state was at a 45-year low.

The baggage hall in Terminal 2 remained closed on Saturday , although all other areas of the airport reopened and flights resumed, with delays, after a 16-hour shutdown.

Airport authorities said they were trying to reunite 20,000 bags and personal items abandoned during the chaos with their owners. The last of thousands of passengers stranded for hours on planes or the tarmac were evacuated from the airport by early Saturday and were taken by bus to the Port Everglades cruise terminal where many had to spend the night.