The do-it-yourself model was professionalized and has been really deployed around the world by ISIS, inspiring dozens of people to carry out attacks in their name,” one expert tells NBC News.

The president vowed “very serious retaliation” to Saturday’s ambush, in which U.S. Iowa National Guard members Sgt. Edgar Brian Torres Tover, 25, and Sgt. William Nathaniel

Last Saturday, an Islamic State infiltrator ambushed a meeting between American soldiers and their local counterparts in a Syrian desert town. Two members of the Iowa National Guard and a U.S. civilian interpreter were killed in the assault—whose perpetrator may or may not have known that the subject of the meeting was how to counter ISIS.

Deadly Syria attack kills two U.S. service members, intensifying pressure on Trump to reassess counter-ISIS strategy and partnership with Ahmed al-Sharaa.

The US struck multiple targets in Syria on Friday that the US military tied to ISIS as retaliation for the recent attack that killed two American service members there, according to two US officials.