“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.