CHICAGO (AFP) - The man who killed five US troops in Tennessee was acting alone, the FBI said on Wednesday, adding it is too early to tell if he was "radicalised" or motivated by mental illness.
Four US marines and a sailor were killed after Mohammad Youssuf Abdulazeez, 24, opened fire on two military centres in Chattanooga before dying in a shootout with police on Thursday.
Authorities have said they are treating the case - reminiscent of other shooting rampages at US military installations - as an "act of terrorism."
"At this time, we are treating him as a home-grown violent extremist," FBI special agent Ed Reinhold told reporters.
"We believe he acted on his own on that day."
Nearly 1,000 FBI agents are pursuing hundreds of leads as they try to understand what motivated Abdulazeez, a naturalised US citizen who was born in Kuwait.
His family has spoken of a history of mental illness and drug abuse and said they sent Abdulazeez to stay with family in Jordan in the hope of getting his life back on track.
Investigators are examining his overseas travel and his online history to see if he came into contact with extremists who may have encouraged him to carry out the attack.
"At this point it's too early in the investigation to determine whether or not he had been radicalised," Reinhold said.
"We are pursuing that as a possibly but it's too early to tell."
Reinhold urged the media to be "responsible when talking about the case" and "not to jump to conclusions about what happened."
Officials also provided a more detailed description of the incident.
Shortly before 11am, Abdulazeez opened fire on a military recruiting centre from his car, wounding one Marine.
He sped away before police could arrive and then crashed through the gate of a nearby naval reserve training centre where 20 Marines and two Navy sailors were inspecting their equipment.
One of the men had a gun and tried to shoot him before he entered the building, but Abdulazeez - who was armed with an assault rifle and two handguns - shot him first.
He then made his way through the building, shooting at anyone he could see. Some of the Marines "willingly ran back into the fight" after getting their comrades to safety, Major-General Paul Brier said.
They tried to provide cover while others climbed a fence at the back of the complex to get away from the shooter.
Then the police arrived and followed the sound of gunfire.
They managed to take down the shooter within three to five minutes after he arrived, Reinhold said.
Rear-Admiral Mary Jackson praised the "absolutely amazing" officers who "reacted and responded that day with such veracity and determination that they most certainly, most certainly saved lives."