A former FBI interpreter reportedly ran away to marry an Isis terror leader before realising her mistake and coming back to the US to turn herself in.

Daniela Greene travelled to Syria where she was wed to German national Denis Cuspert, who rapped under the moniker Deso Dog in his homeland, before changing his name to Abu Talha al-Almani in Syria.

Videos featuring the one time musician — whose primary role is to attract Germans to the terror group — show him on bloody battle fields and threatening former President Barack Obama.

Just a month after her departure, Greene realised her mistake and headed back into the US, according to CNN.

Greene used online tools like Skype to get close to Cuspert before she lied to her FBI employers to travel to Syria, according to Federal Court documents seen by the broadcaster, which also said that she tipped him off to the fact that he was under investigation.

On her return to the US she admitted what she had done and was sentenced to two years imprisonment.

“It's a stunning embarrassment for the FBI, no doubt about it,” John Kirby, a former State Department official told CNN.

Most westerners trying to go to an Isis region in Syria risk “getting their heads cut off," he added. "So for her to be able to get in as an American, as a woman, as an FBI employee and to be able to take up residence with a known ISIS leader, that all had to be coordinated.”

Greene’s story was kept secret when she first returned from Syria. Looking to avoid contaminating the investigation through allowing the case to go public with major media organisations, a federal court sealed most of the documents pertaining to her case.

Once prosecutors determined that Greene’s cooperation with them had finished they asked that the judge unseal some of the documents.

People familiar with Greene said that they hadn’t seen anything that would lead them to think she would defect from the FBI. She had undergone an extensive vetting period for her job with the agency, after completing a Master’s degree in history.

At the time of her departure, she had an American husband who she told that she was going to visit her parents in Munich, Germany. Instead went to Istanbul where she contacted Cuspert.

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But after joinng him in Syria, she quickly determined she had made a mistake.

Greene reportedly quickly began sending messages to someone she knew back home discussing the option to return to the US and cooperate with any investigation into her actions. She returned in August 2014 and was promptly arrested.

Greene was ultimately given two years in prison but she has since been released.