The Iraq war veteran who killed five persons at the Fort Lauderdale airport in the US state of Florida "came specifically to carry out this horrific attack", authorities have said.

Esteban Santiago, 26, cooperated with investigators during an interview that lasted several hours overnight, Xinhua news agency quoted George Piro, special agent in charge of the FBI office in Miami, told the media on Saturday.

"We have not identified any triggers that would have caused this attack," the Federal Bureau of Investigation official said.

According to the official, the suspect landed at the airport with the intention of carrying out the deadly attack.

The investigators were probing whether mental illness played a role in the latest shooting rampage on January 6 that also left eight persons injured.

The FBI has not ruled out terrorism, Piro said, noting it was too early to draw a conclusion. "We continue to look at all avenues, all motives."

The suspect, who flew into the airport on Friday afternoon, went to the baggage claim and started firing indiscriminately.

After a round of shooting, Santiago dropped the handgun, lay ground, faced down, spread eagle and waited for being arrested with no resistance, witnesses said.

Aviation passengers in the US, if declared, are allowed to transport unloaded guns and ammunition in their checked baggage, according to the US Transportation Security Administration.

The attack immediately raised alarm over US aviation security, which has already been tightened since 9/11 attacks.

Santiago spent nine years of service in the National Guard which included one 10-month tour of Iraq, the US military confirmed.

His last military assignment was in Alaska where he served as a member of the Alaska Army National Guard until August when he was discharged for unsatisfactory performance.

His relatives said he had a history of mental health problems especially after returning from Iraq and received psychological treatment last year.

"Only thing I could tell you was when he came out of Iraq, he wasn't feeling too good," his uncle Hernan Rivera told the media.