An FBI interpreter with top clearance traveled to Syria and married the ISIS terrorist she had been assigned to investigate.

In an exclusive report, CNN News unravels the strange case of FBI interpreter Daniela Greene, 38, who went to Syria in 2014 and married the brutal terrorist who had appeared in ISIS propaganda videos holding human heads.

Her husband was Germany-born Denis Cuspert, who rapped under the name Deso Dogg. In Syria, he was known as Abu Talha al-Almani.

He was considered one of the most violent operatives in Syria, including singing songs of praise to Osama bin Laden, threatening Barack Obama with a throat cutting gesture, and holding a bloody head in a recruiting video.

Within weeks of their marriage, Greene, born in Czechoslovakia and raised in Germany, realized marrying Cuspert was a mistake and fled back to the US, where she was arrested and eventually served two years in prison.

She pleaded guilty to making false statements about terrorism and agreed to help the FBI. She was released last summer. Critics say the FBI went light on her, and even people who merely tried to join ISIS have served much longer terms.

CNN said Americans convicted of crimes involving ISIS have served, on average, slightly over 13 years in prison.

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