Story highlights Senate Judiciary Committee chairman asks FBI to explain how an employee with top-secret clearance married ISIS terrorist.

FBI and Justice Department officials asked to brief the committee no later than May 24.

(CNN) A leading US senator on Wednesday asked FBI officials to explain how a rogue employee with top-secret clearance traveled to Syria without their knowledge and married a prominent ISIS terrorist she had been assigned to investigate.

Senator Charles E. Grassley, the chairman of the Judiciary Committee, sent a letter to top FBI and Justice Department officials seeking more information about how the embarrassing national security breach occurred and how they responded to it.

Grassley's letter was in response to an exclusive CNN report last week about the previously unknown saga of Daniela Greene, an FBI translator.

Greene traveled to Syria in 2014 without the FBI's knowledge and married Denis Cuspert, a former German rapper turned ISIS recruiter. Greene told the terrorist he was under investigation by the FBI, according to federal court records.

Greene, 38, returned to the US within weeks of marrying Cuspert and began cooperating with federal authorities. She was sentenced to two years in prison and was released last summer. The details of her cooperation remain shrouded in court-ordered secrecy.

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