The letter was in response to a request by Rep. Adam Smith of Washington state, the top Democrat on the House Armed Services Committee, who questioned the ongoing investigations in light of cuts to the military budget and reports, some written by Republicans, largely clearing the military of any wrongdoing.
"The total cost of compliance with Benghazi-related congressional requests sent to the department and other agencies is estimated to be in the millions of dollars," the Pentagon said.
For example, retired Gen. Carter Ham, the former commander of U.S. Africa Command, has briefed or testified before congressional panels five times over two years, and yet both the Armed Services Committee and House Oversight and Government Reform has asked Ham to submit to additional interviews.
Congressional Republicans have been relentless in investigating the attack, arguing that the Obama administration misled the American people about a terror attack during the heat of the presidential campaign. The GOP is determined to press ahead, especially since the assault on the mission occurred during Hillary Rodham Clinton's tenure as secretary of state.
Clinton is a potential 2016 presidential candidate.
Smith subsequently wrote to Rep. Howard "Buck" McKeon, R-Calif., chairman of the committee, asking that the panel end its involvement with "this witch-hunt."
"More than any other committee in Congress, this committee should understand the financial strain on the department of defense, which is being made worse by these ongoing and ridiculous investigations," Smith wrote.
Claude Chafin, a spokesman for McKeon, said the chairman appreciated Smith's concerns, and added, "it is important that the committee see this oversight effort through to its conclusion."
An independent review in the aftermath of the attack faulted the State Department and security at the compound. A Senate Intelligence Committee report in January said the attack could have been prevented, blaming the State Department, the military and U.S. intelligence.
That report also pointed at Stevens, saying the State Department ended a deal with the military to have a special operations team provide extra security in Libya, and that Stevens twice refused an offer to reinstate the team in the weeks before the attack.
The military also is criticized in the report for failing to respond more quickly on the night of the assault.
But that report as well as a February 2014 report by the Republicans on Armed Services said there was no order to military personnel not to aid those in Benghazi as some Republicans have suggested.
"There was no 'stand down' order issued to U.S. military personnel in Tripoli who sought to join the fight in Benghazi," said the GOP majority on House Armed Services. "However, because official reviews after the attack were not sufficiently comprehensive, there was confusion about the roles and responsibilities of these individuals."
The bipartisan Senate Intelligence report said it had "reviewed the allegations that U.S. personnel, including in the intelligence community or Defense Department, prevented the mounting of any military relief effort during the attacks, but the committee has not found any of these allegations to be substantiated."
Separately, four House Republicans wrote to Homeland Security Secretary Jeh Johnson seeking answers on a draft regulation to lift a longstanding prohibition on Libyans attending flight schools in the United States.
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