Henry, now an executive at the cyber security firm CrowdStrike, said Americans or "U.S. Persons" -- non-citizens who have lived here and have certain legal rights -- radicalized to violent Islamist extremism are "a minority," but the FBI's highest counterterrorism priority.
"Once they get that into their blood, it's a threat," Henry said.
At least 50 young American men have been tracked to the al Qaeda group fighting in Somalia, al-Shabaab, where a young man from the small town of Daphne, Alabama, Omar Hammami, became a top commander.
"Our main objective, one of the things we seek for in this life of ours, is to die as martyrs," Hammami explained in one video, among numerous he made to help al-Shabaab draw Westerners into the fight.
More than 15 U.S. citizens have been killed fighting with al-Shabaab, and at least 20 remain unaccounted for in Somalia.
A new American voice in the Somali terror organization, the as yet unidentified "Abu Ahmed al-Amriki," appeared in a February video, said his countrymen should fight Western governments in Afghanistan, Somalia and Mali.
"America is going down and the Caliphate is rising," he said, brandishing an AK-47.
Hammami -- who remains committed to violent jihad despite a falling out with al-Shabaab leaders -- is now one of the five Americans with U.S. rewards ranging from $1 to $5 million on their head because of their alleged Al Qaeda leadership positions. Others are men from Waukesha, Wisconsin, Brooklyn, New York and Buffalo, New York.
For security reasons, the details of those 100 or so individuals under surveillance inside the U.S. are closely guarded, and both U.S. and European officials say they're focused on Westerners joining al Qaeda in places such as Syria, which is relatively easy to enter.
Asked via Twitter last March about those like him, who turn against America by joining al Qaeda affiliates such as al-Shabaab in Somalia, Hammami did not deny the threat they pose.
Somalia has "many muhajirs from U.S. And dangerous. True," tweeted @abumamerican, an account believed by U.S. officials to be Hammami's.