WASHINGTON (AFP) - Mohammad Youssuf Abdulazeez appears to be law enforcement's worst nightmare - a home-grown, under-the-radar Islamic militant sympathiser unknown to authorities until he strikes.
From Sydney to Paris to Washington, the threat of the "lone wolf" keeps Western intelligence officials up at night, as they try to figure out better ways to home in on potential attackers - and catch them before it's too late.
"This is the event we've been most worried about, and then it happened," Michael McCaul, chairman of the House Homeland Security Committee, said Friday.
"We can only stop so much. They only have to be right one time, one per cent," he told reporters.
The Soufan Group, a New York-based strategic intelligence firm, said Friday the Chattanooga attack "bears the now too-familiar features of recent terror trends."
While Abdulazeez may have been a lone gunman, it said, "helping him pull the trigger... was the ideology of Bin Ladenism, embraced by the members and supporters of the so-called Islamic State in Iraq and Syria (ISIS) and other terrorist groups."
The FBI has so far been careful to say it was too early to ascribe a motive to Abdulazeez, and said there was nothing concrete "that directly ties him to an international terrorist organisation."
But investigators were reportedly looking into a trip the Kuwait-born US citizen made last year to Jordan, to see if he came in contact with any extremists.
"While federal and local law enforcement officials work to determine affiliation and motivation, the source of inspiration might be less difficult to determine," the Soufan Group said, noting the "relentless social media messaging by groups like the Islamic State to attack whenever, however, and wherever possible."
The Federal Bureau of Investigation has repeatedly voiced its concerns about the threat posed by lone wolves, who are swayed by extremist propaganda on the Internet.
FBI director James Comey said last week that the ISIS group - seemingly behind a plot in France to strike a military base - is using Twitter and encrypted messaging platforms to draw new followers and persuade them to murder.
"There's a device, almost a devil on their shoulder all day long saying, 'Kill, kill, kill, kill'," Comey told lawmakers on Capitol Hill.
HOW TO STOP THEM?
Federal and local law enforcement are at a loss as to how to prevent lone wolf attacks, when the assailants often communicate very little - or not at all - with others, and act on their own without outside instructions.
"We certainly didn't have any indication that he was a threat or that yesterday something was going to happen," Chattanooga Mayor Andy Berke said Friday of Abdulazeez, who was described by acquaintances as "kind" and "all-American."
One of the FBI's top priorities is developing better systems to identify individuals who could fall prey to militant groups.
Informants and undercover agents are a key tool in that battle.
The FBI said last week that it had disrupted a number of plots to kill Americans, some of them tied to the July 4 Independence Day holiday, and arrested more than 10 people over suspected links with the Islamic State group.
And the Justice Department has announced multiple arrests in recent weeks of individuals suspected of providing material support to ISIS fighters.
Several experts have highlighted the ideological nature of the battle at hand - made all the more difficult by an identity crisis striking some young Muslims since the Sept 11, 2001 attacks.
"They have craved answers, seeking purpose and belonging," said Farah Pandith, a former Bush and Obama administration official focused on Muslim outreach and countering extremism who is now at the Council on Foreign Relations.
In testimony before US lawmakers, Pandith suggested finding "credible, local voices" in Muslim communities in America to "inoculate... against extremist techniques and appeal."
"I'm talking about helping parents to understand extremist tactics so that they can educate their children about this threat," she said, adding that mental health professionals also needed training to spot possible radicalisation.
"We haven't approached the ideological war with the same resources or respect we did the physical war," she lamented.