Extremists returning home fan security fears

President Barack Obama leaves after speaking about the economy, Iraq, and Ukraine, Thursday, Aug. 28, 2014, in the James Brady Press Briefing Room of the White House in Washington, before convening a meeting with his national security team on the militant threat in Syria and Iraq. (AP Photo/Evan Vucci) less President Barack Obama leaves after speaking about the economy, Iraq, and Ukraine, Thursday, Aug. 28, 2014, in the James Brady Press Briefing Room of the White House in Washington, before convening a meeting ... more Photo: Evan Vucci, Associated Press Photo: Evan Vucci, Associated Press Image 1 of / 1 Caption Close Extremists returning home fan security fears 1 / 1 Back to Gallery

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.

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 up to 100 Americans who have gone to fight for extremist armies in Syria and now Iraq.

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, Sept. 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 e-mails or phone calls can be intercepted.

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.

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.