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.”