Dozens of people from the U.S. who fought in Syria have returned home and are under FBI surveillance, but American officials fear that they haven't identified all of them, several senior officials told ABC News in interviews beginning last October.
The senior officials said that more than 50 "U.S. persons" -- a designation that covers both natural-born and naturalized citizens as well as those who have lived in the U.S. -- have returned here after battling Syrian President Bashar al-Assad's regime in the Middle Eastern nation's bloody civil war. One of the senior counter-terrorism officials went further, saying the actual number of returning U.S. fighters from Syria is classified but is "much higher" than 50.
Not all of those who have returned are considered "jihadis" who adhere to the anti-U.S. violent ideology espoused by the late al Qaeda leader Osama bin Laden, but many are suspected of such sympathies, officials say.
Al Qaeda-aligned jihadi commanders in Syria screen new American arrivals in the ranks of foreign fighters to recruit those with clean passports who have the capability to conduct future operations against the West, two national security officials told ABC News.
One of the officials compared that process of selection to how the U.S. military screens raw recruits for Special Operations Forces qualification courses.
FBI Director James Comey said Thursday the threat is one of his "greatest concerns."
"My concern is that people can go to Syria, develop new relationships, learn new techniques and become far more dangerous, and then flow back," Comey told reporters.
Previous estimates put the number of Americans in the Syrian conflict at 16, but researcher Aaron Zelin at the Washington Institute for Near-East Policy in a report last month said as many as 60 from the U.S. may have fought among an estimated 11,000 foreign militants in Syria.
Only one American, Muslim convert Nicole Lynn Mansfield, 33, is known to have been killed in the Syrian war, though her daughter has insisted her mother was not a terrorist. Mansfield participated in protests against Israel in Michigan before joining combatants in Syria.
Several other reports of Americans killed in Syria fighting with al Qaeda-linked resistance groups have not been verified by the FBI, a spokeswoman told ABC News last week.
One "martyrdom" video supposedly featuring threats against his homeland by an alleged American killed with an al Qaeda group, "Abu Dujana al-Amriki," is considered a likely hoax by the Assad regime, which has capitalized on U.S. jihadis joining Salafist extremists.
Counter-terrorism officials in the United Kingdom and other Western European countries also have privately discussed with their American counterparts their difficulty in identifying citizens or residents of their nations who have slipped into Syria.
It is the easiest war zone for foreign fighters to reach since the Russians faced the insurgency in Afghanistan three decades ago, officials say, which makes tracing the volunteers' travel highly challenging if they're not already on watch lists.
Groups of foreign fighters -- many hardcore Islamist jihadis -- slip into Syria by the hundreds every month through Turkey. They often meet in places such as nearby Bulgaria and make their way there, often by car, said one senior U.S. official.
ABC News Justice Correspondent Pierre Thomas and producer Jack Cloherty contributed to this report.