US federal investigators say a gunman who killed four marines in a shooting spree in Tennessee had no known links to international terrorism.
Mohammad Youssuf Abdulazeez, 24, was shot dead by police after he attacked a military recruitment centre and a naval reserve centre in Chattanooga.
Local prosecutors are investigating the attacks - in which three people were also injured - as "domestic terrorism".
However, the FBI says Abdulazeez's motive is unclear.
Sources close to the case said investigators were looking into a trip made to the Middle East by Abdulazeez last year, including a period spent in Jordan.
Thomas Sullivan from Massachusetts and Skip Wells from Georgia were named as two of the Marines killed in the attack. The other two names are yet to be confirmed.
A police officer and a marine corps recruiter were also wounded, along with a female sailor who remains in a serious condition in hospital.
President Barack Obama described the attacks as "heartbreaking".
They unfolded at 10:45 local time (14:45 GMT) on Thursday.
The suspected was seen pulling up in a rented Ford Mustang convertible outside a shopping centre and opening fire, spraying the offices of a military recruiting centre with bullets.
April Grimmett, who was working at a nearby hair salon, watched the shooting through the window of the shop.
"We heard the [shots]. It was very loud and very fast," she told CNN. "I could not believe how many bullet holes were in that door. It was insane."
At the scene - Gary O'Donoghue, BBC News, Tennessee
As dawn broke over Chattanooga, the serried ranks of television satellite trucks, police cars and FBI vehicles were still in evidence outside the recruitment centre on Lee Highway - scene of the first shooting on Thursday.
Floral tributes lay on the ground, as the Tennessee governor blinked into television lights and promised to do all he could to help the families of the victims.
In truth, he and the FBI are just at the beginning of trying to piece together what drove Mohammad Youssuf Abdulazeez to open fire here and then drive seven miles to kill four US Marines at a navy reserve base.
But progress is clearly being made. A cautious Chattanooga mayor, Andy Berke, told me they were getting snippets from the Muslim community, suggesting that the blurry picture of this young man was starting at least to become a little more clear.
Army spokeswoman Kelli Bland said four US Armed Forces recruiters had been in the building at the time of the shooting, but no one was injured.
The gunman then left the recruitment centre and drove some six miles (10km) to a naval reserve training centre, officials said.
There he fatally shot the four marines and wounded the sailor before being shot and killed himself in a firefight with police.
Employees of nearby businesses spoke of hearing repeated gunshots as police swarmed the area, before it went silent.
Police have sealed off the area around the house in which the gunman lived as they attempt to piece together what led to the attacks.
Eyewitnesses said two women were handcuffed and led away from the house in the Hixson suburb of Chattanooga.
Abdulazeez was reportedly born in Kuwait and had Jordanian citizenship, but was also a naturalised US citizen.
He had attended high school in a Chattanooga suburb and graduated from the University of Tennessee with an engineering degree.
Those who knew him have spoken of their shock at hearing of his involvement.
"He was a good kid... They're good people," said one neighbour. "I've never had any kind of conflict with them."
US officials said he had not been known to federal law enforcement before the attacks, though he had been arrested locally earlier this year for driving under the influence of alcohol.
"We are looking at every possible avenue, whether it was terrorism - whether it was domestic, international - or whether it was a simple, criminal act," FBI special agent Ed Reinhold said.
A blog reportedly set up by Abdulazeez was found by the SITE Intelligence Group, a jihadist monitoring organisation, shortly after the attacks.
In a post from Monday, the writer says "this life is short and bitter" and that Muslims should not let "the opportunity to submit to allah... pass you by".