Eric Harroun, 30, of Phoenix is charged with conspiring to use a rocket-propelled grenade outside the United States while allegedly fighting with the al-Nusra Front, a 16-month-old Salafi jihadi paramilitary extremist group that is part of the opposition Free Syrian Army seeking to oust President Bashar Assad.
The group, also called Jabhat al-Nusra, has claimed responsibility for nearly 600 attacks in Syria since November 2011 and controls parts of northern Syria.
The U.S. State Department designated it a terrorist group in December for suspected ties to al-Qaida in Iraq, which is reputed to want to marginalize Shiite Muslims and establish a "pure Islamic state" in Syria. Assad's Alawite sect is an offshoot of Shiite Islam.
Al-Qaida in Iraq was formed in reaction to the U.S.-led invasion and occupation of Iraq that toppled the regime of secular dictator Saddam Hussein.
The charge against Harroun does not include aiding a terrorist group. It simply says he violated a U.S. law making it illegal to use a weapon of mass destruction outside the United States.
A conviction for such a violation is prison "for any term of years or for life," the law says, adding that if using such a weapon causes someone's death, the conviction can include the death sentence.
An RPG is a shoulder-fired anti-tank weapon system that fires rockets equipped with explosive warheads.
Harroun, who left the Army in 2003 on full disability pay after a truck accident, allegedly entered Syria in January and "participated in attacks led by the al-Nusra Front," states the federal criminal complaint filed Thursday at U.S. District Court in Alexandria, Va.
In a Jan. 26 video Harroun "speaks directly to the camera and states: 'Bashar Assad, your days are numbered. ... Where you go we will find you and kill you,'" the complaint states.
Harroun allegedly told FBI agents in voluntary interviews at the U.S. Embassy in Istanbul, Turkey, he fought with al-Nusra for 25 days, engaging in seven to 10 battles in which he was assigned to an "RPG team," the complaint states, adding he allegedly said he used an RPG to take down a tower on "at least one occasion."
Harroun was last interviewed by the FBI in Istanbul Monday and was arrested Wednesday shortly after flying into Washington Dulles International Airport, officials said.
Harroun appeared in court Thursday when the complaint was filed. U.S. Magistrate Judge Theresa C. Buchanan set a detention hearing for Tuesday.
Neither Harroun nor his court-appointed lawyer could immediately be reached for comment.
Notable deaths of 2014 [PHOTOS]