A U.S. Army veteran has been arrested for allegedly fighting alongside an al Qaeda group in Syria and then bragging about it online.

Phoenix man Eric Harroun, who served in the Army from 2000 to 2003, was arrested Wednesday upon returning to the U.S. from Turkey, where he had described to FBI agents his bizarre journey to the front lines of Syria's civil war with fighters from the al-Nusra Front, a designated terrorist organization also referred to as al Qaeda in Iraq.

According to charging documents, Harroun traveled to Turkey in November 2012 and eventually found people there who agreed to help him slip into Syria to help the rebels in January 2013.

Eric Harroun/Facebook

Once in Syria, he linked up with al Nusra and allegedly participated in several battles against pro-government forces. In later voluntary interviews with FBI agents in Turkey, Harroun claimed to have shot at least 10 people, but said he was unsure if he killed anyone.

Throughout his ordeal, Harroun appears to have posted pictures of himself online in military fatigues with his fellow fighters and weapons, including a rocket-propelled grenade. In one post, he reportedly claimed to have downed a helicopter. He also appeared in online videos threatening Syria's President Bashad al-Assad, court documents said.

The court documents say Harroun admitted online and to the FBI agents that he had fought with the al-Nusra Front, but claimed that he hates al Qaeda and was only trying to help topple the Assad regime. The U.S. government has repeatedly called on Assad to step down and recent news reports allege the U.S. is helping the rebels acquire weapons from friendly regional governments.

In an interview with Fox News earlier this month, Harroun said he was welcomed by al-Nusra.

"Getting into al-Nusra is not rocket science," he said, according to Fox News. "It just takes balls and brains."

A follow-up article published in Foreign Policy, written by the same reporters as the Fox News report, described the journalists' curious interactions with Harroun, who is described as unpredictable, inconsistent and, at times, suddenly hostile.

"Pinning Harroun down is never easy," the Foreign Policy article says. "At times he will provide very specific details about himself, while at other times he becomes more reserved, preferring to not comment or flat-out denying his previous statements - only to retract his retractions."

The criminal complaint against Harroun says federal officials believe they have probable cause to believe Harroun "conspired to use a weapon of mass destruction," meaning the RPG. He will remain in custody pending a preliminary hearing in early April, federal officials said.

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