WASHINGTON -- The case of Mehdi Nemmouche haunts U.S. intelligence officials.
Nemmouche is a Frenchman who authorities say spent 11 months fighting with the Islamic State group in Syria before returning to Europe to act out his rage. On May 24, prosecutors say, he methodically shot four people at the Jewish Museum in central Brussels. Three died instantly, one afterward. Nemmouche was arrested later, apparently by chance.
For U.S. and European counterterrorism officials, that 90-second spasm of violence is the kind of attack they fear from thousands of Europeans and at least 100 Americans who have gone to fight for extremist armies in Syria and now Iraq.
Tom Sanderson, a terrorism expert at the Center for Strategic and International Studies, told CBS News that it's very hard to tell which Americans who have traveled to Syria since 2011 might come home and pose a threat.
"To know that would be indicative of a very deep level of intelligence on the person, to be able to know that he, most likely, would have the inclination to attack the U.S. upon return or to attack a U.S. equity overseas," Sanderson said.
The Obama administration has offered a wide range of assessments of the threat to U.S. national security posed by the extremists who say they've established a caliphate, or Islamic state, in an area straddling eastern Syrian and northern and western Iraq, and whose actions include last week's beheading of American journalist James Foley. Some officials say the group is more dangerous than al Qaeda. Yet intelligence assessments say it currently couldn't pull off a complex, 9-11-style attack on the U.S. or Europe.
However, there is broad agreement across intelligence and law enforcement agencies of the immediate threat from radicalized Europeans and Americans who could come home to conduct lone-wolf operations. Such plots are difficult to detect because they don't require large conspiracies of people whose emails or phone calls can be intercepted.
The 2013 Boston Marathon bombings were like that, carried out by radicalized American brothers Dzhokhar and Tamerlan Tsarnaev acting on their own. So was the 2010 attempt to bomb New York's Times Square by Faisal Shahzad, who received training and direction in Pakistan but operated alone in the United States.
On Friday, Britain raised its terror threat from "substantial" to "severe," its second highest level, citing a foreign fighter danger that made a terrorist attack "highly likely." The U.S. didn't elevate its national terrorist threat level, though White House press secretary Josh Earnest said the administration was closely monitoring the situation. Homeland Security Secretary Jeh Johnson said Friday that U.S. authorities aren't aware of any "specific, credible" threats to the U.S. homeland from the group.
So far, Nemmouche is the only foreign fighter affiliated with the Islamic State of Iraq and Syria (ISIS) who authorities say returned from the battlefield to carry out violence, and some scholars argue the danger is overstated. But nearly every senior national security official in the U.S. government - including the attorney general, FBI director, homeland security secretary and leaders of key intelligence and military agencies - has called foreign fighters in Syria and Iraq their top terrorism worry.
"While we have worked hard over the last year and a half to detect Westerners who have gone to Syria, no one knows for sure whether there are those who have gone there undetected," said John Cohen, who stepped down in July as the Homeland Security Department's counterterrorism coordinator.
One reason it's so hard to track which Westerners have gone to join the fight is because of a lack of robust intelligence collection in the region, according to Seth Jones, a former U.S. counter-terrorism official.
"The U.S. had a pretty good intelligence collection capability in Afghanistan in Iraq, including human intelligence collection, including people on the ground, including special operators," said Jones, who is now with the Rand Corp. think tank, told CBS News. "I don't think it's a highly resourced effort right now."
Unlike al Qaeda militants in Pakistan and Yemen, American and European passport holders who have secretly gone to fight in Syria can travel freely if they have not been identified as terrorists. U.S. authorities are sifting through travel records and trying to identify the foreign fighters, but they won't see all of them.
An American from San Diego, Douglas McAuthur McCain, was killed this week in Syria, where, officials say, he was fighting with ISIS. The U.S. is investigating whether a second American also was killed.
McCain is one of several Western Muslims over the last two years who proved themselves willing to kill or die for extremist groups or help them win new recruits. The names of many more remain secret in the files of U.S. intelligence agencies, but here are others that are public:
-Moner Mohammad Abusalha, an American who grew up a basketball fan in Vero Beach, Florida, killed 16 people and himself in a suicide bombing attack against Syrian government forces in May. U.S. officials say he was on their radar screen but acknowledge he traveled from Syria to the United States before the attack without detection. Had he attacked in the U.S. instead of Syria, it's unclear whether he would have been stopped.
-Two brothers from East London, Hamza Nawaz, 23, and Mohommod Nawaz, 30, pleaded guilty in May to attending a terrorist training camp in Syria. They were caught on the return trip home with ammunition. In an unrelated case, Mashudur Choudhury, 31, was also convicted in London of traveling to a terrorist camp in Syria.
-Three Norwegian residents were arrested in May and accused of having fought with ISIS.
-Eight men, including a former Guantanamo Bay detainee, were arrested in June by Spanish authorities and charged with recruiting for ISIS.
Of the thousands of foreign fighters who've flocked to Syria, many have fought with the al Nusra front, an al Qaeda affiliate and rival to ISIS. The group poses its own threat, American officials say, but poses less of a threat than does ISIS, whose battlefield successes have made it a stronger draw for foreign fighters than any Jihadist group in recent history. It has seized advanced military equipment and has millions of dollars in cash.
The U.S. government has positively identified a relatively small number of Americans - fewer than 12 - who have joined ISIS, but precise numbers are unavailable and intelligence assessments, while educated, are still estimates due to limited U.S. intelligence in Syria, CBS News correspondent Bob Orr reports.
Nemmouche, who has a long criminal record, allegedly killed two Israeli tourists outside the Brussels museum entrance with a .357 Magnum revolver. Then he walked inside, removed an assault rifle from a gym bag and shot two museum employees in the face and throat, prosecutors say.
He was caught six days later during a random customs inspection of a bus from Amsterdam. With him were the murder weapons, authorities say, and a sheet scrawled with the name of ISIS. He had intended to film the attack with a wearable video camera, authorities say, though it wasn't working that day.
Abusalha, the 22-year-old Vero Beach suicide bomber, was recorded in a series of videos before his attack. In one of them, he addresses the U.S. public in American-accented English.
"You think you are safe? You are not safe," he said. "We are coming for you, mark my words."