The man who shot and killed five US troops in Tennessee suffered from depression, his family said Saturday in condemning the "heinous act of violence."
Four Marines and a sailor were killed in the attack -- which authorities are treating as "an act of terrorism" -- before the gunman, Mohammad Youssuf Abdulazeez, died in a shootout with police.
"There are no words to describe our shock, horror and grief," Abdulazeez's family said in a statement cited by an NPR public radio journalist.
"The person who committed this horrible crime was not the son we knew and loved," it added.
"For many years, our son suffered from depression. It grieves us beyond belief to know that his pain found its expression in this heinous act of violence."
Expressing its condolences to the families of the victims, the Abdulazeez family also vowed to continue to cooperate with law enforcement.
Investigators are seeking to determine a motive behind Thursday's rampage on two military centers in Chattanooga.
The Federal Bureau of Investigation has asked foreign intelligence agencies to help trace Abdulazeez's movements and activities abroad, and analysts are monitoring his activity on social media.
A law enforcement official told the New York Times investigators were looking into a text message Abdulazeez allegedly sent to a friend hours before the shooting to probe possible motives.
The text reportedly included a verse: "Whosoever shows enmity to a friend of Mine, then I have declared war against him," according to the newspaper.
"Every one of our resources are being devoted to this investigation," Chattanooga Mayor Andy Berke told CNN.
But the FBI warned against jumping to conclusions, after Michael McCaul, chairman of the House of
Investigators are searching for evidence that the man who killed four US Marines may have contacted extremists either online or during overseas travel but so far have found nothing, the FBI said Friday.
Mohammad Youssuf Abdulazeez, 24, opened fire on two military centres in Chattanooga on Thursday, gunning down 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 an "act of terrorism”.
After a senior US lawmaker suggested the evidence pointed to an attack inspired by Daesh, the FBI warned against jumping to conclusions about a possible motive.
"At this time, we have no indication that he was inspired by or directed by anyone other than himself," FBI special agent Ed Reinhold said of Abdulazeez, a naturalised US citizen born.
The FBI has asked foreign intelligence agencies to help trace his movements and activities abroad, and analysts will be tracing his activity on social media.
"We obviously want to know what his thoughts were and who else he was associating with," Reinhold told reporters.
Michael McCaul, chairman of the US House Homeland Security Committee, said the attack was inspired by the Daesh, also known by its acronym ISIS.
In the wake of the shooting, Defense Secretary Ashton Carter asked for recommendations by the end of next week on how to "ensure the safety of service members and civilians at military installations," Pentagon spokesman Peter Cook said.
Foreign travel in spotlight
Investigators were looking at Abdulazeez's foreign travel, with a reported trip to Jordan last year of particular interest.
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 Friday confirming that Abdulazeez was born in Kuwait and had last visited in 2010 for three weeks.
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 fueled fears of so-called "lone wolf" actors -- attackers with no known affiliation to an extremist group who are difficult to detect.
"What keeps me up at night is the one case we don't know about," McCaul said. "This is the event we've been most worried about, and then it happened."
US media reports described Abdulazeez as having had a traditional American upbringing, including participation on school sports teams.
He graduated from the University of Tennessee at Chattanooga with a degree in engineering in 2012.
Scott Schrader, who coached Abdulazeez in mixed martial arts, told CNN he "seemed like the all-American kid."
There was evidence, however, that he came from a troubled family.
Divorce papers filed by his mother and obtained by AFP allege that his father beat his wife and five children.
His father was also reportedly investigated for ties to a terrorist group, but ultimately cleared.
Abdulazeez had a brush with the law in April when he was arrested for driving under the influence, local media reported.
He briefly worked at a nuclear power plant in Ohio in May 2013, but was fired for not meeting minimum employment requirements, said a spokeswoman for Perry Nuclear Power Plant operator FirstEnergy.
'It hit really hard'
The victims were identified as Gunnery Sergeant Thomas Sullivan, Staff Sergeant David Wyatt, Sergeant Carson Holmquist and Lance Corporal Squire Wells.
Wells joined the Marines last year. The other three men had each served at least two tours of duty in Iraq or Afghanistan.
Shaken residents gathered at memorials to mourn them.
"It hit really hard," said Ansley Chilton, a student at the University of Tennessee-Chattanooga.
"I wanted to come to honor the victims, especially since it was so close to home."
Chattanooga Police Chief Fred Fletcher heaped praise on the officers who helped prevent further loss of life.
He described how an officer who was shot in the ankle was dragged to safety by his colleagues "and bravely returned fire to ensure that he was safe and the gunman remained engaged."
The officer, 17-year-old Dennis Pedigo, "is doing well, he is in good spirits," a family statement read.