The Iraq war veteran accused of killing five people and wounding six others at a South Florida airport traveled there specifically to carry out the attack, the FBI said on Saturday.
Esteban Santiago, 26, opened fire with a 9mm semi-automatic handgun in a baggage claim area after flying from Alaska. He flew to Fort Lauderdale with a gun in his checked luggage, according to FBI agent George Piro.
Investigators do not know what motivated him to target the airport.
"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," he said.
A handgun that was taken from Santiago during a mental health evaluation in Alaska last year was returned to him a month later, according to police.
Santiago had walked into an FBI office in Anchorage in November in agitated state the FBI said, leaving his newborn child in a car outside, as well as his handgun.
He told the FBI he was hearing voices and believed he was being controlled by a US intelligence agency that wanted him to watch jihadi videos by the extremist group, Islamic State.
"Santiago was having terroristic thoughts and believed he was being influenced by ISIS," Anchorage Police Chief Chris Tolley told the news conference.
But the authorities deemed he was not a threat and the gun was returned in December. It was unclear if it was the gun used in Friday's attack at Fort Lauderdale-Hollywod international airport.
Federal law only allows for gun rights to be taken away from someone on mental health grounds if they have committed a crime or are officially declared to be mentally unfit.
It is legal to pack firearms in passenger luggage in the United States as long as the guns are kept in a locked container as checked baggage.
According to his brother, Santiago lived in Alaska where he was undergoing psychological treatment. He was born in New Jersey but moved to Puerto Rico as a young child.
He served in the U.S. military n Iraq from April 2010 to February 2011, and later joined the Alaska National Guard before he was discharged for "unsatisfactory performance" last year, officials say.
Esteban Santiago returned from a tour in Iraq a changed man, his aunt told CNN. "His mind was not right," the aunt, Maria Ruiz Rivera, said.
He talked about the destruction he witnessed, the killing of children and visions that haunted him. "He seemed normal at times, but other times he seemed lost. He changed," she said.
Among the victims was Olga Woltering, a devout Catholic in her 80s, from Marietta, Goergia, who was on her way to a cruise to celebrate her husband's 90th birthday. Another victim, 57-year-old Iowa surveyor, Michael Oehme.
A third victim was named as Terry Andres, 63, a volunteer fireman from Virginia Beach, Virginia.