The findings of the Pentagon's inspector general are contained in a draft report that has not been publicly released but was obtained by the independent watchdog group, the Project on Government Oversight (POGO).
The probe throws a potentially embarrassing light on President Barack Obama's administration, suggesting the White House was keen to help filmmakers tell a flattering story even as it aggressively investigated government officials and journalists over possible leaks of sensitive information.
The inspector general (IG) found that Panetta, then CIA director, had disclosed the "top secret" information two years ago at a ceremony at the spy agency's headquarters honoring those who took part in the raid that killed Osama Bin Laden, according to the draft report posted online by POGO.
The event was attended by a "Hollywood executive" and screenwriter Mark Boal, who was working on the movie "Zero Dark Thirty," which recounts the Bin Laden manhunt, it said.
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"According to the DoD (Department of Defense) Office of Security Review, the individual's name is protected from public release -- under federal law."
The inspector general report adds: "Director Panetta also provided DoD information, identified by relevant Original Classification Authorities as TOP SECRET..."
The probe had been requested by the Republican chairman of the House Homeland Security Committee, Peter King of New York, who was concerned the administration may have revealed classified information and secret tactics to Hollywood filmmakers.
The Pentagon declined to comment on the POGO article.
It was unclear why the Pentagon inspector general's office, which is supposed to hold the Defense Department accountable, has not released its findings on the cooperation given the filmmakers, according to POGO.