Syria Militants Said to Recruit Visiting Americans to Attack U.S. WASHINGTON - Islamic extremist groups in Syria with ties to Al Qaeda are trying to identify, recruit and train Americans and other Westerners who have traveled there to get them to carry out attacks when they return home, according to senior American intelligence and counterterrorism officials. These efforts, which the officials say are in the early stages, are the latest challenge that the conflict in Syria has created, not just for Europe but for the United States, as the civil war has become a magnet for Westerners seeking to fight with the rebels against the government of President Bashar al-Assad. At least 70 Americans have either traveled to Syria, or tried to, since the civil war started three years ago, according to the intelligence and counterterrorism officials - a figure that has not previously been disclosed. The director of the F.B.I., James B. Comey, said Thursday that tracking Americans who have returned from Syria had become one of the bureau's highest counterterrorism priorities. "We are focused on trying to figure out what our people are up to, who should be spoken to, who should be followed, who should be charged," Mr. Comey said in a meeting with reporters, without referring to specific numbers. I mean, it's hard for me to characterize beyond that. It's something we are intensely focused on. Fearing that the handful of Americans who have returned to the United States pose a threat because they may have received extensive training and jihadist indoctrination, the F.B.I. is conducting costly round-the-clock surveillance on a small number of these individuals, according to the officials. "We know Al Qaeda is using Syria to identify individuals they can recruit, provide them additional indoctrination so they're further radicalized, and leverage them into future soldiers, possibly in the U.S.," said a senior counterterrorism official, who, like half a dozen other top intelligence, law enforcement and diplomatic officials interviewed for this article, spoke on the condition of anonymity because he did not want to be identified discussing delicate national security issues. In Europe, where larger numbers are leaving for Syria, officials share the same concern and are working closely with American authorities to coordinate measures to stem the flow and track those who return. Analysts say at least 1,200 European Muslims have gone to fight since the start of the civil war. In a confidential memo on Nov. 26, Gilles de Kerchove, the European Union's counterterrorism coordinator, warned that "the first returnees have come back, and there are cases where individuals continue traveling back and forth." Most of the Americans who have traveled to Syria are still there, the officials said, though a few have died on the battlefield. Nicole Lynn Mansfield, 33, of Flint, Mich., a convert to Islam, was killed last May while with Syrian rebels in Idlib Province. Another American, Eric G. Harroun, a former Army soldier from Phoenix, was indicted in Virginia by a federal grand jury last year on charges related to allegations that he fought alongside the Nusra Front, one of the Syrian opposition groups linked to Al Qaeda. In September, he pleaded guilty to a lesser charge involving conspiracy to transfer defense articles and services, and was released from custody. Mr. Harroun's involvement was hardly a secret. Last February, he bragged about his role, posting a photo on his Facebook page saying, "Downed a Syrian Helicopter then Looted all Intel and Weapons!" American officials say their concerns about the recruitment and training of Americans are based on intelligence gleaned from passenger travel records, human sources on the ground in Syria, intercepted electronic communications, social media postings and surveillance of Americans overseas who have expressed interest in traveling to Syria. The authorities are also trying to identify Americans traveling there by scouring travel data that the European Union has been providing to the Department of Homeland Security as part of a 2011 agreement. While the main goal of the Nusra Front and the Islamic State of Iraq and Syria, another group with ties to Al Qaeda, remains toppling Mr. Assad's government, American officials said the groups had carved out enough space and influence to begin building the apparatus to conduct attacks outside Syria. Despite the United States" use of powerful surveillance tools and drone attacks on Qaeda leaders in places like Pakistan and Yemen, Mr. Comey said in the meeting with reporters that he was worried about a "metastasizing Al Qaeda threat" in Africa and the Middle East. "We've had great success against core Al Qaeda in the Af-Pak region," Mr. Comey said, referring to Afghanistan and Pakistan, "but at the same time, in the ungoverned or poorly governed spaces in Africa and around the Middle East, we see a resurgence of Al Qaeda affiliates." The group's attempts to create a pipeline into the United States suggest that it is still not deterred from trying to follow through on its most lofty, and difficult, goal of carrying out an attack on American soil. "That Al Qaeda would like to get operatives into the homeland or in Western Europe has been a persistent theme over the past several years," said one senior law enforcement official. Indeed, the extremists" efforts in Syria are taking a page from the playbook of Al Qaeda and its associates in Pakistan, where jihadist talent spotters have sought to identify, recruit and train American citizens or residents before they return home. Both Najibullah Zazi, a former coffee cart operator who unsuccessfully plotted to detonate backpack bombs on the New York City subway, and Faisal Shahzad, a Pakistani-born American convicted in the failed Times Square bombing of 2010, received training in Pakistan. The challenge of identifying Americans who are trying to travel to Syria is one of the greatest challenges that the United States Customs and Border Protection's National Targeting Center in Dulles, Va., has faced since it was created in October 2001. But American law enforcement and counterterrorism officials have dealt with a similar threat over the past few years from roughly three dozen Somali-Americans who have traveled to Somalia to fight there. The F.B.I., local law enforcement agencies and Somali community leaders have overcome initial hurdles to cooperate in identifying individuals who could pose a threat. But unlike those in the Somali group, largely young men from a few communities like Minneapolis and Columbus, Ohio, the Americans heading to Syria pose a much thornier challenge because they are "a much larger group of people traveling there for a wider array of reasons," the senior law enforcement official said. The cross section of folks we're aware of is very broad. Richard Stanek, the sheriff of Hennepin County, Minn., where Minneapolis is, said he had been contacted by several federal officials seeking advice on how to deal with this more diverse potential threat. But his advice carries caveats. "Our experiences are different than what we're seeing with Syria," said the sheriff, who is also president of the Major County Sheriffs" Association, which represents the nation's 77 largest sheriff offices. The same indicators aren't necessarily there.