An FBI translator traveled to Syria and married a German rapper-turned so-called "Islamic State" (IS) operative she was assigned to investigate, US broadcaster CNN has reported in a special investigation.

FBI translator Daniela Greene was already married and held a top secret security clearance when she secretly traveled through Turkey to Syria in the summer of 2014 to meet and briefly marry Denis Cuspert, a Ghanaian-German also known by his stage name "Desso Dog," CNN reported.

Known in IS circles as "the German," Cuspert called for terror attacks in Europe and is believed to have been injured in a US airstrike in 2015. The fate and whereabouts of the former Berlin-based rapper are unknown. 

Greene was born in the former Czechoslovakia and raised in Germany where she married a US Army soldier. The soldier later brought her to the United States where she studied history and eventually joined the FBI field office in Detroit, CNN reported.

Greene free under plea deal

After spending two months in Syria during which Greene told Cuspert he was under investigation as a Specially Designated Terrorist, she "had a change of heart" and returned to the United States where she was arrested, CNN said, citing recently unsealed court documents. 

Greene served two years in prison under a plea deal while her case was kept from the public until prosecutors asked for her court record to be unsealed. CNN noted that while the FBI had "page after page" of press releases detailing IS-related arrests and court sentences ranging from five to 15 years, Greene's case received no attention.

"Unlike other terrorism related cases Daniela Greene's arrest and plea deal would receive no publicity at all from the Department of Justice, the case quietly hidden, court records sealed for months, even after her case became a matter of public record, still silence," CNN investigative reporter Drew Griffen said in a television report.

Greene cooperated with authorities in a plea deal and is currently free on parole.