Eric Harroun, 30, served in the US Army from 2000-2003, and allegedly fought in Syria with the al-Nusra Front.

BBC: US ex-soldier 'fought in Syria with terror group'

Eric Harroun, 30, of Phoenix was arrested Tuesday night by the FBI at a hotel near Washington Dulles International Airport in Virginia.

CNN: U.S. ex-soldier charged with using RPG for al Qaeda group

The WMD law also applies to "any national of the United States who, without lawful authority, uses or threatens, attempts, or conspires to use a weapon of mass destruction outside of the U.S.." In March 2013, former U.S. soldier Eric Harroun was charged under this part of the statute for allegedly conspiring to use a rocket propelled grenade while fighting with an al Qaeda affiliated group in Syria.

CNN: WMD: From A-bombs to pressure cookers