A top Defense Department official is criticizing the House Benghazi committee’s investigation as wasteful and inefficient, while questioning whether the panel’s final report will be based in fact.

In a letter to the committee Thursday, Assistant Secretary of Defense for Legislative Affairs Stephen Hedger charged Republican investigators with employing a variety of “unproductive” tactics,  including “continuous threats” to subpoena Pentagon employees, “multiple and changing requests” to the department and interviews that prompt speculation by witnesses.

“While I understand your stated intent is to conduct the most comprehensive review of the attack and response, Congress has as much an obligation as the Executive Branch to use federal resources and taxpayer dollars effectively and efficiently,” Hedger wrote.

“The Department is working diligently to accommodate your staff’s multiple and changing requests; however, we are concerned by the continuous threats from your staff to subpoena witnesses because we are not able to move quickly enough to accommodate these new requests,” Hedger wrote. “Subpoenaing our service members, when the Department is working diligently to accommodate your requests and when no service member has refused to appear voluntarily, is unfair to our uniformed men and women.”

The letter underscores the challenges facing the two-year-old House Select Committee on Benghazi, which has been dogged by criticism from its right and left. Panel chairman Trey Gowdy (R-S.C.) has promised to release the final investigative report “before summer,” but critics are skeptical it will happen.

In recent months, Rep. Gowdy has touted “immense” progress in the investigation, citing the receipt of new documents from the Obama administration and more than 90 witness interviews in total. He has not provided further detail.

“This letter is further proof the Benghazi Committee is conducting a thorough, fact-centered investigation,” committee spokesman Matt Wolking said in a statement.

“It’s unfortunate it took the threat of subpoenas for the Pentagon to make witnesses available earlier this year,” Wolking said. “This delayed the committee from learning a tremendous amount of new information from several witnesses, and when they refer the committee to others in the department, the responsible thing to do is to interview them. What is DOD so afraid of? Why are they supposedly unable to find their own employees?”

The panel’s ranking Democrat accused Republicans of “dragging out their sham of an investigation closer to the election.”

“The Department of Defense has a critical job to do, which is to keep our nation safe from those who would do us harm,” Rep. Elijah Cummings (D-Md.) said in a statement. “But Republicans continue to squander millions of taxpayer dollars chasing right-wing conspiracy theories and forcing Pentagon officials to waste their time on this partisan fishing expedition.”

Hedger’s letter provides a rare glimpse into tensions between the Benghazi panel and the Defense Department.

The letter described a sudden “crescendo of requests” for interviews by the panel, including with witnesses Hedger described as duplicative or unnecessary, despite statements that the probe was nearing an end.

“While we understand that investigations evolve over time, it is unfortunate that the Committee has identified the need for these interviews only now,” Hedger wrote. “The number and continued pace of these requests since February 2016 are in tension with your staff’s statements that the Committee expects to finish its investigation in the near term. Perhaps because of this conflict, the Committee’s requests are accompanied by unrealistic timelines for the Department.”

Hedger wrote that Pentagon witnesses have been “asked repeatedly to speculate or engage in discussing on the record hypotheticals . . . regardless of the interviewee’s actual knowledge or expertise.”

“This type of questioning poses the risk that your final report may be based on speculation rather than a fact-based analysis,” he wrote. “. . . I would respectfully request that you ensure pending interviews remain focused on obtaining facts rather than encouraging speculation.”

Gowdy has argued that the investigation has been stymied by the Obama administration from the beginning.

“The committee still does not have records we requested over a year ago, and we are still waiting for some witnesses to be made available for interviews,” Gowdy said in a statement on April 8. “As soon as possible, we will release our report and interview transcripts so everyone can see the evidence for themselves, and I’m confident the value and fairness of our investigation will then be abundantly clear to everyone.”