Federal officials say 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 U.S. as battle-hardened security risks.

The State Department says it has no estimates of how many Americans have taken up weapons to fight military units loyal to Syrian President Bashar Assad in the 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 a couple dozen. The IHS group says Al Qaeds-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 U.S. 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 this month, Sen. Thomas Carper, D-Del., 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 Director 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 this 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. The FBI refused to say whether it's directed agents to increase efforts to stop Americans bound for Syria.

In the case of Sheikh, his North Carolina home isn't 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-Qaida magazine who was killed in a drone attack in Yemen.

Sheikh is charged with planning to assist a group the State Department has declared a terrorist organization. It's 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 U.S.

For five months this year, Sheikh didn't 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.

In August, Sheikh commented to an undercover FBI employee's posts on a Facebook page promoting Islamic extremism. The two struck up an online relationship, the affidavit said. Sheikh told the informant he planned to trek to Syria to join "a brigade in logistics, managing medical supplies." Days later, Sheikh said he'd bought a one-way ticket to travel to Turkey in hopes of making contact with people who would get him to Syria.

Sheikh said he backed out because "he could not muster the strength to leave his parents," the affidavit said. Sheikh said he had traveled to Turkey last year hoping to join the fight in Syria, but became dispirited by his experience with people who claimed to be part of the U.S.-backed Free Syrian Army. After Sheikh expressed online support for Jabhat al-Nusrah and interest in traveling to the war zone, the FBI employee suggested Sheikh contact a person with the group — another FBI informant.

Sheikh made contract, describing Jabhat al-Nusrah as the most disciplined group of anti-Assad fighters, the affidavit said. "I'm not scared," Sheikh wrote, according to the affidavit. "I'm ready."

Two federal public defenders appointed to represent Sheikh are barred by local court practice from discussing their cases, spokeswoman Elizabeth Luck said. Sheikh's father, Javed Sheikh, said his son was falsely accused but that he trusts U.S. courts to find the truth.

A federal magistrate ruled that Sheikh should be detained until his trial because there was clear evidence that he wouldn't appear if released on bond and that there was a "serious risk" to the community if he were freed.

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Basit Sheikh's arraignment is scheduled for January. He could face up to 15 years in prison and a $250,000 fine if convicted.

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