(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.
Greene declined to discuss her case with CNN.
Grassley has been one of the FBI's fiercest critics over the years, criticizing misconduct and disciplinary problems and pushing for greater legal protection for internal whistle-blowers who report abuses.
In his letter addressed to Deputy Attorney General Rod J. Rosenstein and acting FBI Director Andrew McCabe, Grassley asked a series of questions about Greene's actions, FBI oversight, and the Justice Department's decision to argue for a reduced sentence.
"How and when did the FBI become aware that Greene had engaged in a romantic relationship with Cuspert," Grassley asked in his first and most over-arching question. "Please explain how this relationship developed without the FBI's knowledge."
Grassley also sought details about the FBI's travel screening procedures, asked what steps had been taken to ensure "a similar incident will not happen in the future," and wanted to know whether any FBI employees had been disciplined in the wake of Greene's case.
Finally, the Republican senator asked prosecutors to explain the decision "to argue for [Greene] to receive a reduced sentence."
In court papers filed in US District Court in Washington D.C., Assistant US Attorney Thomas Gillice characterized Greene's conduct as "egregious," deserving of "serious punishment."
Even though Greene's "conduct skirted a line dangerously close to other more serious charges," the prosecutor argued she should receive a lighter sentence because of her cooperation.
Greene's two-year sentence was much less than punishments given to other defendants charged with terrorism-related crimes.
Even failed attempts to travel to Syria and join ISIS have led to much stiffer prison sentences. The charge of material support for terrorism, for example, carries a potential 15-year sentence.
A Justice Department official, however, told CNN Greene's sentence was "in line" with similar cases, but declined to cite examples.
In his argument for a reduced sentence, Gillice noted that Greene immediately began cooperating with authorities. Her cooperation was "significant, long- running and substantial," he wrote.
Shortly after Greene pleaded guilty, Cuspert was deemed a "Specially Designated Global Terrorist" by the U.S. State Department.
A bulletin on the agency's website cited Cuspert's oath of loyalty to ISIS and his appearance in a video in which he held a severed human head he said belong to man executed for opposing the group.
It called Cuspert "a willing pitchman for [ISIS] atrocities," and said he appears to be a recruiter with a special emphasis on recruiting German speakers.
Greene, who is fluent in German, was hired by the FBI as a contract linguist in 2011. She was assigned to a counter-terrorism investigation involving Cuspert in early 2014.
The publicly accessible material in her case file does not explain how their relationship began.
Grassley asked FBI and Justice Department officials to provide answers to his committee no later than May 24.