Afghanistan: U.S. Identifies Citizens Joining Rebels in Syria, Including ISIS -- WASHINGTON — American intelligence and law enforcement agencies have identified nearly a dozen Americans who have traveled to Syria to fight for the Islamic State in Iraq and Syria, the militant group that the Obama administration says poses the greatest threat to the United States since Al Qaeda before the Sept. 11, 2001, attacks. -- As ISIS has seized large expanses of territory in recent months, it has drawn more foreign men to Syria, requiring more American and European law enforcement resources in the attempt to stop the flow of fighters, senior American officials said. And as a result of the increasing numbers of men, ISIS is now recruiting foreign women as jihadist wives. -- .ISIS has become more attractive to would-be militants because, unlike Al Qaeda, it has seized territory that it rules by strict Islamic law. “ISIS is able to hold itself up as the true jihad,” said a senior American official. “They’re saying: ‘Look at what we are doing, what we’re accomplishing. We’re the new face. We’re not just talking about it. We’re doing it.’ ” -- ISIS’ attraction to some is based on its reputation for brutality. On Thursday, that reputation grew worse when it was revealed that it had waterboarded four hostages early in their captivity — including the American journalist James Foley, who was beheaded this month. -- Over all, American intelligence officials said the number of Americans who have joined rebel groups in Syria — not just ISIS — had nearly doubled since January. The officials now believe that more than 100 Americans have fought alongside groups there since the civil war began three years ago. -- The agencies have been able to specifically identify Americans fighting for ISIS based on intelligence gathered from travel records, family members, intercepted electronic communications, social media postings and surveillance of Americans overseas who had expressed interest in going to Syria, the officials said. -- Many more Europeans have joined the fight against President Bashar al-Assad — more than 1,000, according to many estimates. The British government has identified about 500 of its citizens who have gone to Syria, according to a senior British official. About half have returned to Britain, and a small number have died on the battlefield, the official said. -- Senior American officials acknowledge that as the conflict in Syria and Iraq drags on, it is becoming harder to track Americans who have traveled there. In many instances, the American law enforcement and intelligence agencies are learning that Americans are there only long after they have arrived. --- In the latest example of how difficult it is for the United States to track its citizens, the F.B.I. on Thursday was trying to verify reports that two more Americans had been killed fighting for ISIS in Syria. -- Since the beginning of the Syrian conflict in 2011, at least four Americans have died fighting for rebel groups, including Douglas McAuthur McCain, 33, a Minnesota man who was fighting for ISIS when he was killed last weekend by a rival group backed by the United States. -- Another challenge that the intelligence and law enforcement authorities say they face is a difference from previous conflicts: The Americans who have traveled to Syria to fight have little in common. The conflict has attracted both men and women, including some who were raised as Muslims and others who converted from Christianity, and they have come from different parts of the United States.