An international investigation is under way today to find out whether the Marine Corps Reserve lance corporal who was arrested early Friday morning for carrying suspected bomb making materials near the Pentagon is simply an unstable, misguided young man, or a cold-blooded lone wolf terrorist.
"We don't know what a lone wolf, al Qaeda-inspired operative looks like. We don't know where they hang out, we don't know really what motivates them," former FBI agent Jack Cloonan said. "So when you don't know that, you've got a talent pool of people that is so huge, it stresses law enforcement. We just don't know what they look like and what they want to do."
The United States was lucky in this incident, because police were able to arrest the man in question, 22-year-old Ethiopian-American Yonathan Melaku, who was recently charged with breaking into 27 cars in suburban Washington. But so-called lone wolf terrorists are generally very difficult to catch.
"It gets very close to that whole issue of profiling," Cloonan said. "We don't want to say as law enforcement that we're going to look for every Arab male age 21 to 35. We really can't do that. Profiling is not that effective in this regard."
FBI investigators have been dissecting Melaku's life since his arrest, but as of yet have found no links to terrorist organizations, although he was carrying pro-al Qaeda literature.
"We do believe at this time that this individual acted alone," said Brenda Heck, a special agent in charge of counterterrorism for the FBI.
But while he may have been acting alone, the recent appearance of a "hit list" on a jihadi web site that names 40 prominent figures from government, the U.S. military and the media who should be attacked, has officials concerned about people who might have no formal links to al Qaeda but still be inspired by their rhetoric.
Among the names on the hit list, which includes photographs of the targets and biographical information, are a member of Congress, Pentagon officials, a conservative pundit, executives of an American company involved in the production of drone aircraft, and two prominent French executives.
According to a bulletin circulated by the FBI, the hit list appeared on the website Ansar al-Mujahideen after one poster highlighted Al Qaeda leader Adam Gadahn's call in a June 3 message for lone wolf attacks on American public figures and corporate institutions.
"When al Qaeda ... [puts] a hit list out ... I think that they're trying to issue an open-ended fatwa, a religious order, for people to go act on their own," Cloonan said. "The person who is sitting out there, the lone wolf ... we just don't know. That's what stresses law enforcement ... because we just don't know."
With the anniversary of 9/11 approaching, we can expect to see more of these attacks, according to Cloonan.
"Al Qaeda and other likeminded groups are adapting to what we've done," he said. "We've degraded their leadership, we've degraded their operational capability, they will take what we give them -- the lone wolf, the single operative, is probably the modus operandi they're relying on."
Melaku was arrested around 2 a.m. Friday, when an army policeman confronted him at Arlington Cemetery. It is unclear what he was doing there in the middle of the night, but his actions set off alarm bells.
Melaku fled, but the police ran him down and found four zip-lock bags with a substance labeled ammonium nitrate, a key bomb-making material, in his backpack, officials said. Also inside were spent 9mm ammunition and a notebook containing the words "al qaeda," "Taliban rules," "mujahidin" and "defeated coalition forces," according to police.
Police worried that Melaku had planted bombs at the cemetery and the Iwo Jima memorial located just a mile from the Pentagon. All traffic was shut down around the Pentagon while authorities raced to Melaku's suburban home in Alexandria, Va., to search for bomb making material.
Searches of Melaku's home and car turned up no explosives, and the material in Melaku's backpack tested negative as a potential explosive, sources said.
Melaku was arrested last month for smashing windows and stealing valuables from 27 cars in Leesburg, Va., and charged with four counts of grand larceny.
A man by the same name and birth month was also arrested in Fairfax County for reckless driving and failure to stop. He pleaded guilty and paid a $200 fine for the former charge and $30 for the second charge.
Melaku, who joined the Marine Corps Reserve on Sept. 4, 2007, according to the FBI, had been awarded the National Defense Service Medal and the Selected Marine Corps Reserve medal.
ABC News' Pierre Thomas and Bianna Golodryga contributed to this report.