US leaders paid tribute Saturday to the four Marines and the sailor killed last month in a lone gunman's attack on military facilities here, vowing justice against violent extremists.

Vice President Joe Biden, US Defense Secretary Ashton Carter and other dignitaries joined families of the victims for a solemn memorial service a month after the July 16 attack in this river city flanked by the Appalachian mountains.

A sorrowful Biden eulogized the five in strikingly personal terms, alluding to the recent loss of his own son, Beau Biden.

"I wish I were not here. I have some sense of how hard it is for you to be here," he told the families.

Likening their sons and husbands to biblical warriors, he said: "The men we honor today gave the same answer to Isaiah: Send me."

America, he said, "Never kneels, never bends, never cowers, never stands down. (America) endures, responds, and we always overcome."

FBI investigators say Mohammad Youssuf Abdulazeez acted alone, but authorities are unsure whether he was inspired by radical ideology or driven by mental illness as his family has suggested.

"The meaning of their killing is yet unclear, what combination of disturbed mind, violent extremism and hateful ideology was at work, we don't know," said Carter.

But he added: "The few who threaten or incite harm to Americans -- violent extremists or terrorists wherever they are -- very surely, no matter how long it takes, will come to feel the long arm and hard fist of justice."

The Kuwaiti-born 24-year-old former engineering student was shot dead by police to end the rampage at two separate military facilities.

- Review of protective measures -

Carter has ordered the military services to review their procedures for protecting American troops inside the United States and to take immediate steps to beef up security at recruiting stations.

The service chiefs have until Friday to present their plans.

The victims were unarmed when Abdulazeez attacked, firing first at a military recruiting station from his speeding car and then crashing through the gate of a nearby naval reserve facility.

Armed with an assault rifle and a handgun, Abdulazeez went through the building shooting at whomever he could find, according to authorities.

Marines in the facility helped many of those inside escape over a back fence, before police arrived on the scene and killed Abdulazeez.

The shocking attack in broad daylight in a sleepy heartland city drew angry demands that US military personnel be allowed to go to work armed.

The military pushed back against that idea as impractical, but Carter has pressed the service chiefs to come up with detailed plans for protecting recruiting stations and other domestic military reserve facilities.

Carter stressed that military recruitment has continued undeterred despite the threat highlighted by the Chattanooga killings.

But recruiting stations are often located in shopping malls or other heavily trafficked public places, which poses a dilemma for the military: it needs access to the public to fill their ranks, but the high visibility also can make recruiters an easy target.

US officials have repeatedly raised fears of lone-wolf attacks by individuals inspired by groups like the self-proclaimed Islamic State, and have stepped up arrests of Americans seeking to travel to Syria or conspiring to carry out attacks at home.

The FBI has investigated a trip Abdulazeez took to Jordan last year, but his family says he was sent to stay with relatives to get away from friends who were a bad influence.

A naturalized American citizen, Abdulazeez was a resident of Tennessee.   

Before the memorial service in a basketball arena here, Biden and Carter met privately with the families of the victims -- Gunnery Sergeant Thomas Sullivan, 40, Staff Sergeant David Wyatt, 35, Sergeant Carson Holmquist, 25, Navy logistics specialist Randall Smith, 26, and Lance Corporal Squire Wells, 21.

Several of them had served tours of duty in Iraq and Afghanistan, and several left behind young wives and small children.

Their names were called out, twice each, in a ceremonial "last roll call" by members of their unit.