WASHINGTON Hundreds of U.S., British and Canadian citizens who have trained with Islamic State fighters trying to carve out their own state in Iraq and Syria pose a "very serious threat" to the United States, a top Republican lawmaker said Sunday.
Representative Mike Rogers, chairman of the House Intelligence Committee, urged the Obama administration to aggressively prosecute U.S. citizens who had trained overseas as it weighed options for how to respond to escalating violence by Islamic State militants in Iraq.
He said an attack on the Jewish museum in Brussels in May had shown a new push by extremist groups to carry out attacks on Western targets, and growing efforts to recruit backers on social media.
"I'm very concerned because we don't know every single person who has gone and trained and learned how to fight," Rogers told "Fox News Sunday."
The United States carried out three air strikes on Saturday against Islamic State fighters near the besieged Shi'ite town of Amerli in northern Iraq and airdropped more than hundred additional bundles of humanitarian aid to civilians trapped there, the Pentagon said.
President Barack Obama last week authorized the new military action, broadening U.S. operations in Iraq amid an international outcry over the threat to Amerli's mostly ethnic Turkmen population. Washington is seeking to build an international coalition to fight against Islamic State forces.
Rogers said he believed hundreds of U.S. citizens had trained with Islamic State at least once, in addition to an estimated 500 British citizens and hundreds more from Canada, with their passports essentially allowing them "free travel" to the United States to carry out extremist attacks.
"The chances of error are greater than our ability to track every single area. It's a very serious threat," he said, noting that U.S. intelligence agencies were also tracking "pretty serious" threats of planned attacks in the West by al Qaeda.
Concerns about Americans associating with Islamic State militant fighters have grown in recent weeks after the deaths of two U.S. citizens in Syria.
U.S. Attorney General Eric Holder said in July dozens of Americans were among roughly 7,000 foreign fighters that U.S. intelligence agencies estimate to be operating in Syria, out of roughly 23,000 violent extremists.
Rogers said the number of Americans who had traveled and trained overseas at least once was considerably higher.
He said U.S. efforts to tamp down on such travel by U.S. citizens needed to be handled carefully to avoid restricting the ability of other Americans to travel overseas.
But aggressive prosecution of those who trained with extremist groups could have a deterrent effect, he said.
Representative Dutch Ruppersberger, the top Democrat on the House Intelligence Committee, echoed Rogers' concerns.
"The biggest threat that I see to the United States right now are Americans and Brits who have passports that have the ability to come into our country without getting a visa," Ruppersberger told CNN's "State of the Union" program.
"We had the suicide American bomber who was radicalized, came home to visit his parents, went back and then killed himself. Now, that could have happened in the United States," Ruppersberger said.
A 22-year-old man from Florida carried out a suicide bombing in Syria's Idlib province in May. A Denver woman was arrested in July on suspicion of trying to fly to Syria to help insurgents, and two men in Texas were arrested on similar charges in June.