Investigators trying to determine why a 24-year-old Kuwait-born Tennessee man gunned down four Marines are looking at his travel to Jordan last year to see whether he was in contact with extremists, reports say.
Mohammad Youssuf Abdulazeez opened fire on two military centres in Chattanooga on Thursday, killing the four Marines and wounding three people before dying in a shootout with police.
Authorities have said they are treating the case - a grisly scene reminiscent of other shooting rampages at US military installations - as one of "domestic terrorism."
The FBI has warned against jumping to conclusions about a possible motive, and said so far, there is no information directly linking Abdulazeez - a naturalised US citizen - to an international terror group.
But investigators are combing through his computer, mobile phone and social media contacts to unearth details about a months-long trip he made to Jordan last year, the New York Times and Wall Street Journal reported on Friday.
"This attack raises several questions about whether he was directed by someone or whether there's enough propaganda out there to motivate him to do this," an unnamed senior intelligence official told the Times.
A US official who asked not to be named confirmed the Jordan trip to AFP.
Bassam Issa, president of the Islamic Society of Greater Chattanooga, told CNN that Abdulazeez had moved overseas to live with relatives about two years ago.
"My understanding is recently he had come back... and was working with a company in Nashville," Issa told CNN.
The Kuwaiti interior ministry issued a statement on Friday confirming that Abdulazeez was born in Kuwait and had last visited in 2010 for three weeks, but was of Jordanian descent.
The gunman had not given authorities any reason to place him under surveillance prior to Thursday's attacks, Chattanooga mayor Andy Berke said.
"We certainly didn't have any indication that he was a threat or that yesterday something was going to happen," Berke said.
Although no motive has been formally established so far, the incident fuelled fears of so-called "lone wolf" actors - attackers with no known affiliation to an extremist group who are extremely difficult to detect.
Abdulazeez graduated from the University of Tennessee at Chattanooga with a degree in engineering.
Scott Schrader, who coached Abdulazeez in mixed martial arts, told CNN he "seemed like the all-American kid."
In an apparent blog post written Monday, Abdulazeez said Muslims should not let "the opportunity to submit to Allah... pass you by," according to the SITE Intelligence Group, which monitors extremist activity on social media.
The FBI said that so far, there had not been any indication that anyone else was involved in the shooting.
"We have no idea at this point what his motivation was behind this shooting," FBI special agent Ed Reinhold said late Thursday.
President Barack Obama called the shootings "heartbreaking" and asked Americans to pray for the relatives of the victims.