US Army veteran Eric Harroun from Phoenix was charged Thursday in Virginia for his actions with a US-designated terrorist group fighting against the Syrian regime of Bashar al Assad.
US District Court files show the 30-year-old is charged with "conspiring to use a weapon of mass destruction — i.e., a rocket propelled grenade — outside the US."
According to the documents, Harroun crossed over the border into Syria from Turkey in January to help aid Syria's opposition rebels.
ABC News said in Syria he joined al Nusra fighters and allegedly took part in a number of battles against pro-government troops.
In January, a YouTube video showed four men with Harroun, who spoke to the camera and threatened Syria's president.
"Your days are numbered, you're going down in flames, you should just quit now while you can," Harroun said. "You're going to die no matter what ... we will find you and kill you." (Foreign Policy wrote a feature on Harroun that can be read here.)
Jabhat al-Nusra, or the al-Nursa Front, has been linked to al Qaeda and the US government has designated it as a terrorist organization.
More from GlobalPost: Syria: One on one with the leader of Jabhat al-Nusra
When Harroun returned to the US on Wednesday, he was arrested at Washington's Dulles International Airport. In an interview with FBI agents, Harroun claimed he was part of an "RPG team."
The FBI affidavit goes on to say:
"As part of that team, Harroun carried three rockets in a back pack along with an AK-47 rifle. Harroun stated he carried by anti-personnel and anti-armor rockets. Harroun stated that he and his team would fire rockets at the defenses of regime forces. Harroun stated that on at least one occasion he hit a tower with a rocket. Other members of Harrouns's RPG team also fired rockets and had successful hits on Syria targets."
If convicted, Harroun could face life in prison, though he is not charged with giving material support to al-Nursa, only conspiring to use a weapon of mass destruction, according to the Washington Post.
He remains in custody pending a preliminary hearing next month.