Federal officials say Americans are joining the bloody civil war in Syria, raising the chances they could become radicalized by al-Qaida-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 in the 3-year-old war that has killed more than 100,000 people. But two sets of London-based analysts put the number of Americans among the al-Qaida-linked fighters at a couple dozen.

A Pakistan-born North Carolina man arrested on his way to Lebanon has become the third U.S. resident this year charged with planning to fight beside Jabhat al-Nusrah. The U.S. considers the radical Islamic organization a foreign terrorist group. The group opposes Syrian leader Bashar Assad.