CHATTANOOGA (REUTERS) - Hours before the Tennessee shooting that killed five US servicemen, the suspected gunman texted his close friend a link to a long Islamic verse that included the line: "Whosoever shows enmity to a friend of Mine, then I have declared war against him."

The friend, who requested anonymity, showed the text message to Reuters on Saturday.

He said he thought nothing of the message at the time, but now wonders if it was a hint at Thursday's attack in Chattanooga.

The suspect, Mohammod Youssuf Abdulazeez, a 24-year-old Kuwaiti-born naturalised US citizen, was killed in a gunfight with police.

The FBI is investigating the shooting as an act of terrorism, but said it was premature to speculate on the gunman's motive.

The rampage has re-ignited concerns about the radicalisation of young Muslim men. Abdulazeez's friends said he returned from a trip to Jordan in 2014 concerned about conflicts in the Middle East and the reluctance of the United States and other countries to intervene.

After the trip, he purchased three assault rifles on an online marketplace and used them for target practice, the friends said.

"He expressed that he was upset about (the Middle East). But I can't imagine it drove him to this," said the friend who received the text message.

Authorities said Abdulazeez sprayed gunfire at a joint military recruiting centre in a strip mall in Chattanooga, then drove to a Naval Reserve Centre about 10km away, where he killed four Marines before he himself was shot dead.

Three other people were injured, including a US Navy petty officer who died from his wounds on Saturday.

The Navy did not give the name of the sailor, but his step-grandmother identified him as Navy Petty Officer 2nd Class Randall Smith of Paulding, Ohio, who left behind a wife and three young daughters.

"He was an awesome young man," Darlene Proxmire told Reuters.

"He loved his wife and children. He loved the Navy."

JORDAN TRIP WAS "EYE-OPENING"

Abdulazeez's friends, who asked not to be identified for fear of a backlash, said he was upset about the 2014 Israeli bombing campaign in Gaza and the civil war in Syria.

"He felt Jordan, Qatar, Saudi Arabia were not doing enough to help, and that they were heavily influenced by the United States," said the friend who received the text message.

Another friend said, "He had always talked about it, but I'd say his level of understanding and awareness really rose after he came back."

Abdulazeez, an engineer, had occasionally smoked marijuana and drank alcohol, and struggled to reconcile that with his faith in Islam, the friends said.

At one point, in 2012 or 2013, he received therapy for his drugs and alcohol use, they said.

"He used it to de-stress, when things were difficult at home, or whatever," the first friend said, adding that tensions between Abdulazeez and his Palestinian parents had upset him.

His parents nearly got divorced in 2009, according to court records.

Abdulazeez also had problems with local youths that sometimes took on a religious and racial tone, the friend said.

"There were rednecks, ignorant people, who sometimes would cause problems. Mo never fought, but he used to get worked up and yell and stuff," he said.

"Afterwards he would calm down, and just say it doesn't matter."

Abdulazeez went to the Middle East in 2010 and visited several countries, according to the friend. He then went to Jordan in 2014 to work for his uncle, and lived with his uncle and his grandparents there, the friend said.

"That trip was eye-opening for him. He learned a lot about the traditions and culture of the Middle East. He said he really enjoyed it and wished to go back some day."

After Abdulazeez returned, he seemed more mellow to his friends, less interested in partying.

"That is part of what drew us closer. He was a guy who wanted to settle down and get his life going. That connected us," the friend said.

GUNS, FAST CARS

Abdulazeez had purchased three guns on armslist.com after returning from Jordan, including an AK-74, an AR-15, and a Saiga 12, his friends said. They said he also owned a 9mm and a .22 calibre hand guns.

Over the past few months, Abdulazeez and his friends practised shooting in the Prentice Cooper state forest near Chattanooga, sometimes two or three times a week.

"He was always interested in guns, since he was young. He started with a BB gun and paintball, and went on from there. We would go out shooting quite often," said the friend who received the text message.

Abdulazeez also liked driving fast in the hills surrounding Chattanooga.

Two nights before the attack, he and some friends went joyriding in Abdulazeez's rented gray convertible Ford Mustang, passing through the towns of Whitwell, Dayton and Jasper.

"Fast car on a rainy night. We were flying, doing tight turns and drifting," said the friend, adding that they returned home at about 3 o'clock in the morning.

"He seemed totally normal. We made plans to hang out on the weekend," he said.

The night before the attack, just after 10pm, the friend received a text from Abdulazeez with this link to a Hadith, or Islamic teaching: http://sunnah.com/nawawi40/3.

For militants and ultraconservative Salafist Sunni Muslims, the Hadith "is usually understood within the context of al-wala wa-l-bara (or) love for Islam and hatred for its enemies," said David Cook, an associate professor who specialises in Islam in the department of religion at Rice University in Texas.

It was unusual for Abdulazeez to send such a link, said his friend, who had been asking Abdulazeez for job advice.

"I didn't see it as a hint at the time, but it may have been his way of telling me something," he said.

He continued to text Abdulazeez that evening and into Thursday but did not get a reply.

The friend said he has been interviewed by the Federal Bureau of Investigation.

He said Abdulazeez had a good paying job and many plans for his life, including possibly starting a computer sales business in Chattanooga.

"He wanted to buy a car. He wanted a video console, to make a man cave - every guy's dream."

He said it was difficult to understand how his friend became the suspect in the rampage.

"The signs just weren't there," he said.

"The only thing I can think of is that it was a combination of things - what is happening overseas, his family problems, maybe some of the issues with the less educated people here. I don't know."

After the shooting, the friend texted Abdulazeez, asking him if he had heard about the attack. "I guess he knew about it before I did."

Investigators are trying to establish if Abdulazeez was part of an organisation or the latest "lone wolf" militant - radicalised US Muslims acting on their own, who President Barack Obama has said pose a greater risk to the country than a large-scale operation.

"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.

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 that "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 authorised 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 neighbours 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 Abdulazeez told the Chattanooga Times Free Press in 2010.

The Islamic Society mosque where Mohammod worshipped cancelled 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.