RALEIGH, N.C. — Federal officials said Americans are joining the bloody civil war in Syria, raising the chances they could become radicalized by Al Qaeda-linked militant groups and return to the United States as battle-hardened security risks.
The State Department said it has no estimates of how many Americans have taken up weapons to fight military units loyal to Syrian President Bashar Assad in the nearly 3-year-old war that has killed more than 100,000 people.
Other estimates — from an arm of the British defense consultant IHS Jane’s and from experts at a nonprofit think tank in London — put the number of Americans at about two dozen. The IHS group says Al Qaeda-linked fighters number about 15,000, with total anti-Assad force at 100,000 or more.
This year, at least three Americans have been charged with planning to fight beside Jabhat al-Nusrah — a radical Islamic organization that the United States considers a foreign terrorist group — against Assad. The most recent case involves a Pakistan-born North Carolina man arrested on his way to Lebanon.
At a Senate homeland security committee hearing last month, Senator Thomas Carper, a Democrat from Delaware, said: ‘‘We know that American citizens as well as Canadian and European nationals have taken up arms in Syria, in Yemen, and in Somalia. The threat that these individuals could return home to carry out attacks is real and troubling.’’
The hearing came about two weeks after the FBI and other officers arrested Basit Sheikh, 29, at the Raleigh-Durham International Airport on charges he was on his way to join Jabhat al-Nusrah. Sheikh, a legal resident of the United States, had lived quietly, without a criminal record, in a Raleigh suburb for five years before his Nov. 2 arrest.
A similar arrest came in April in Chicago. And in September, authorities in Virginia released an Army veteran accused of fighting alongside the group after a secret plea deal.
In August, outgoing FBI chief Robert Mueller told ABC News that he was concerned about Americans fighting in Syria, specifically ‘‘the associations they will make and, secondly, the expertise they will develop, and whether or not they will utilize those associations, utilize that expertise, to undertake an attack on the homeland.’’
Current FBI Director James Comey said last month that he worried about Syria becoming a repeat of Afghanistan in the 1980s, after the Soviet invasion, with foreign fighters attracted there to train.
In the case of Sheikh, his North Carolina home is not considered a breeding ground for terrorist activity.
But Aaron Zelin, who works for both the London-based International Centre for the Study of Radicalisation and the Washington Institute for Near East Policy, notes that Sheikh lived about three hours from the hometown of Samir Khan, the editor of an English-language Al Qaeda magazine who was killed in a drone attack in Yemen.
Sheikh has been charged with planning to assist a group that the State Department has declared a terrorist organization.
It is not illegal for Americans who also hold citizenship in another country to fight in that country’s military. But American citizenship can be lost for voluntarily serving in foreign armed forces hostile to the United States.
For five months this year, Sheikh did not know he was being monitored as he posted messages and videos on Facebook expressing support for jihadi militants fighting Assad’s forces, according to a Nov. 2 sworn affidavit by FBI Special Agent Jason Maslow in support of the warrant to arrest Sheikh.