The suspect in the fatal shootings of four US Marines traveled to Jordan and possibly other Middle Eastern countries for seven months last year, authorities said on Friday, as the investigation focused on any signs of a connection to Islamist militants.

Mohammod Youssuf Abdulazeez, a Kuwaiti-born naturalized US citizen, died on Thursday in a firefight with police after a rampage at two military facilities in Chattanooga, Tennessee.

The 24-year-old engineer traveled to Jordan, from around April to November, US government sources and friends of Abdulazeez in Chattanooga told Reuters. One childhood friend, speaking on the condition of anonymity, said he went there for a job opportunity.

Investigators will try to establish if he was part of an organization or the latest "lone wolf" militant, radicalized US Muslims acting on their own who President Barack Obama has said pose a greater risk to the country than a large-scale operation.

Friends were shocked by the actions of Abdulazeez, who they said lived about 150 miles (241 km) away in Franklin, Tennessee, but had returned to his hometown to visit family for the holy fasting month of Ramadan, which ended Thursday.

"He was a friend of mine, a good Muslim. But there were no red flags, nothing unusual. It is shocking," said another childhood friend, who prayed with him at the Islamic Center of Greater Chattanooga over the past month.

A little more than 24 hours after the shooting, the FBI said it continued to investigate it as an act of terrorism and that it was "premature" to speculate on the motive.

"We are exploring all travel that he has done and we have asked our intelligence partners throughout the world to provide us with any information they may have," Ed Reinhold, FBI special agent in charge, said during a news conference.

Born to Palestinian parents and raised in a Chattanooga suburb, Abdulazeez may have family in Jordan and may have made several stops, said a government source, adding that a visit to Yemen, long viewed as a training ground for Islamic militants, has not been ruled out.

Law enforcement officials have said they are investigating whether Abdulazeez was inspired by Islamic State or similar militant groups. Islamic State had threatened to step up violence during Ramadan.

But the Federal Bureau of Investigation said Friday it had no indication that the attack was linked to that group.

'A HAPPY HOME'

Abdulazeez sprayed gunfire at a joint military recruiting center in a strip mall, riddling the glass facade with bullet holes, then drove to a Naval Reserve Center about 6 miles (10 km) away, where he killed the Marines before he himself was shot dead. Three other people were injured.

He wore a vest that law enforcement officials said may have been used to hold extra ammunition and had two long guns and a handgun.

The Marine Corps identified the four slain Marines as Gunnery Sergeant Thomas Sullivan of Hampden, Massachusetts; Staff Sergeant David Wyatt of Burke, North Carolina; Sergeant Carson Holmquist of Polk, Wisconsin; and reservist Lance Corporal Squire Wells of Cobb, Georgia.

The SITE Intelligence Group, which tracks extremist groups, said Abdulazeez blogged on Monday "life is short and bitter" and that Muslims should not miss an opportunity to "submit to Allah." Reuters could not independently verify the postings.

Investigators believe family or psychological issues may have contributed, according to a government source, who was not authorized to speak on the record.

Years ago, his father, Youssuf Abdulazeez, an engineer who attended Texas A&M University, came under investigation by a Joint Terrorism Task Force for possible connections to a militant group, one source said. But he was cleared of any association with terrorism or wrongdoing.

His son attended high school in a Chattanooga suburb and graduated from the University of Tennessee in 2012 with an engineering degree.

In 2013, he was hired as an engineer at an Ohio nuclear plant and spent 10 days there before he was let go. A spokesman for the FirstEnergy Corp, which owns the plant, did not say why he was dismissed and would not confirm media reports that he had failed a background check.

While friends and the family's neighbors said there were no signs that warned of his rampage, not all was going well for the young man. In April, he was arrested and charged with driving under the influence.

The family also appears to have undergone upheaval in 2009, when the mother, Rasmia Abdulazeez, petitioned for divorce and alleged abuse of her and the children, according to court documents. The suit was dismissed and the couple signed a post-nuptial agreement.

One of the childhood friends said Mohammod's family life was good and called it "a happy home."

But at least one of his four siblings complained of the difficulty they faced being Muslims at their high school, saying they were harassed by fellow students.

"There's this misconception that Islam is a violent religion. Muslims are actually peaceful," a 17-year-old Yasmeen Abdulazeeztold the Chattanooga Times Free Press in 2010.

The Islamic Society mosque where Mohammod worshipped canceled activities to celebrate Eid, marking the end of Ramadan, but called all Muslims to attend a vigil at a Baptist church Friday night.

Islamic Society member Dr. Mohsin Ali told the gathering that Abdulazeez "did his best to spread hatred and division."

"And we will not let that endure," he said to a standing ovation.