U.S. officials estimate over 20,000 foreign fighters aiding extremists

U.S. intelligence officials now estimate more than 20,000 foreign fighters have made their way to the Middle East to join al Qaeda, the Islamic State of Iraq and Syria (ISIS) and other extremist groups, offering an updated assessment of what many have described as the most dangerous threat facing the U.S. The estimate, based on data first obtained by the Associated Press, was confirmed by CBS News.

At least 3,400 of those fighters have come from Western nations, including as many as 150 Americans who have tried to reach -- or succeeding in reaching -- the battlefield. Some of those Americans have been arrested en-route and others have died in battle, but some are actively fighting alongside the extremists in the region.

Many of the foreign fighters are flocking to Iraq and Syria, where ISIS controls a chunk of territory roughly the size of Belgium. In Syria, ISIS fighters are fighting the regime of Syrian President Bashar Assad and a smattering of rebel groups who oppose both the regime and the extremists. In Iraq, Iraqi security forces and Kurdish militias are engaged in a heated ground campaign against the group. In both countries, U.S.-led airstrikes have pummeled ISIS for months.

In prepared remarks for a congressional hearing on Wednesday, House Homeland Security Chairman Michael McCaul, R-Texas, described the ingathering of jihadist elements in the region as "the largest convergence of Islamist terrorists in world history," and he warned that their ranks continue "to grow despite months of airstrikes."

Experts are particularly concerned about the possibility that foreign fighters from Western nations who have thus far evaded detection could use their passports to return home and launch domestic attacks. McCaul said Wednesday that some of the Americans who've enlisted in the terrorist cause "have already returned to our shores."

Nicholas Rasmussen, the director of the National Counterterrorism Center, described the flow of foreign fighters as one of America's "most pressing" security concerns during Wednesday's hearing.

"The rate of foreign fighter travel to Syria is unprecedented," he said. "It exceeds the rate of travelers who went to Afghanistan and Pakistan, Iraq, Yemen, or Somalia at any point in the last 20 years." Rasmussen described how ISIS and other organizations recruit foreign fighters using social media. He also updated lawmakers on the international effort to stem the flow of extremist recruits, highlighting the "increased willingness" of various nations to coordinate enforcement and share information regarding foreign fighters.

President Obama sent an authorization for the use of military force to Congress on Wednesday, asking for broad authority to fight the extremists across the region. Republican leaders on Capitol Hill have said they believe they need to give the president the authority necessary to succeed, but they've suggested it's up to Mr. Obama to make the public case for his request.