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.

Rogue FBI interpreer Daniela Greene, 38 (above), did two years in prison after secretly marrying an ISIS terrorist

Denis Cuspert, (left) known as Abu Talha al-Almani, married an American FBI agent, who fled their relationship after a month

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.

Greene spent two years in prison, much less than the average ISIS sympathizer spends

Greene grew up in Germany and worked for the FBI as an interpreter - she as put on the case of a German terrorist who had joined ISIS, who would go on to become her short-lived husband

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.

He said that an American woman getting into Syria and working her way into the inner sanctum of ISIS, she would have had to have been preapproved by its leaders.

When reached by CNN, Greene only said 'If I talk to you my family will be in danger.'

'She was just a well-meaning person that got up in something way over her head,' said her attorney, Shawn Moore.

Cuspert, above, was a rapper in Germany before he became known for his terror videos for ISIS recruitment, including ones in which he held up freshly-cut off heads

Greene moved to the US as a young adult who'd married an American soldier, and attended college at Cameron University in Oklahoma and graduate school at Clemson University.

In 2011, Dani, as she was nicknamed, became a contract linguist for the FBI.

In January 2014, she was put on the case of a German terrorist known as 'Individual A,' who was Cuspert.

Cuspert was a rapper who had middling success in Germany under the name Deso Dogg, and whose career highlight was opening for rapper DMX in 2006.

In 2010, a near death experience in a car accident saw him quit rapping and turn to Islam. He rapidly accelerated to radicalization, and in 2013, entered Syria.

Meanwhile, Greene, as part of her job, was tracking Cuspert's communications and gained access to several of his online accounts and phone numbers. One Skype number she kept sole access to.

On June 2014, Greene she told the FBI she was going on vacation to visit her family in Germany.

Instead, she flew to Turkey and then Gaziantep, near the Syrian border.

There, she met up with Cuspert and married him, despite still being married to her American soldier husband.

However, by July, the honeymoon period was over. She emailed a friend saying she was having second thoughts.

'I was weak and didn't know how to handle anything anymore. I really made a mess of things this time,' she wrote.

The next day, she continued: 'I am gone and I can't come back. I wouldn't even know how to make it through, if I tried to come back. I am in a very harsh environment and I don't know how long I will last here, but it doesn't matter, it's all a little too late...'

By the end of the month, she was saying 'I wish I could turn back time some days' and how if she returned, she would go to prison.

Meanwhile, her new husband was seen on video beating a corpse with a sandal.

In August, Greene was somehow able to get out of Syria and back to the US, where she was arrested.

In a case that was largely kept sealed and private, Greene pleaded guilty in December and was sentenced to two years in prison.

In October 2015, the Pentagon said Cuspert was killed in an air strike, but nine months later, decided he was alive after all.