According to the letter sent Sunday, the committee's former chief Republican investigator concluded in January that "nothing could have affected what occurred in Benghazi."
In interviews with the then-Secretary of Defense Leon Panetta and his chief of staff, the committee's chief counsel, who recently left the committee, made statements saying he agreed that there was no way the military teams could have reached the complex where the attacks took place in time to save any lives. The investigator, Dana Chipman, is not named in the letter, but he is a retired three-star general who also served as Judge Advocate General for the Army.
During his questioning of Panetta in January, Chipman said, "I think you ordered exactly the right forces to move out and to head toward a position where they could reinforce what was occurring in Benghazi or in Tripoli or elsewhere in the region. And sir, I don't disagree with the actions you took, the recommendations you made, and the decisions you directed."
The top Democrat on the panel, Rep. Elijah Cummings, a Maryland Democrat, and Rep. Adam Smith, a member of the panel who also serves as the ranking Democrat on the House Armed Services Committee, cite Chipman's comments in their response to a recent letter Gowdy sent to Defense Secretary Ash Carter.
In that letter, the GOP chair criticizes the Defense Department for what he says is a lack of cooperation with the investigation. Recently, the panel has asked the department to question witnesses who might have information about whether the military did in fact deploy sufficient resources.
During the committee's investigation, which has spanned two years, nearly 100 witnesses have been interviewed. Gowdy has stressed that the majority of those interviewed by the select committee are people who have not previously testified about Benghazi before other congressional panels.
For their part, Democrats have repeatedly charged the GOP is dragging out its review in an effort to target former Secretary of State Hillary Clinton, who is the front-runner for the 2016 Democratic nomination for president.
The potential new witnesses include one individual who made claims on a talk radio program hosted by Sean Hannity and another who posted on Facebook that he was a crew member who helped scramble jets at an airbase but that the alert to respond never came.
The Pentagon's Assistant Secretary of Defense, Stephen Hedger, complained in a letter to Gowdy in late April that these requests were redundant and required excessive time and expense, and come at what many expect is the end of the investigation.
That Pentagon pushback triggered a blistering response from Gowdy, who wrote to Carter, "I would rather risk interviewing a witness in good faith, who ultimately produces little probative information, than risk not interviewing one who does."
The GOP Chairman also accused the Pentagon of coordinating with Democrats and said the Pentagon staff was wasting taxpayer money "writing partisan, factually deficient letters" to the committee.
In response to the Democrats' letter, the GOP gave CNN a statement from Chipman, who doesn't dispute the quotes attributed to him that the Democrats cite, but says he agrees with Gowdy's efforts.
"If some witnesses refer the committee to other witnesses, the responsible thing to do is interview them. The committee has an obligation to the American people to determine what can and cannot be substantiated, so if an individual makes public allegations about Benghazi, the committee should interview that person."
The Democrats defend the complaints from the Pentagon, saying in their letter that the "evolving demands" are "now putting a strain on the Pentagon that is completely unwarranted, unreasonable, and unjustified."
Matt Wolking, spokesman for Gowdy, ripped the Democrats' letter as "dishonest" saying, "Republicans will continue conducting a thorough, fact-centered investigation that includes all relevant witnesses, regardless of rank."
Democrats point to previous investigations conducted by the House Armed Services Committee, the Intelligence Committee and the Senate Intelligence panel that concluded that when the U.S. consulate in Benghazi came under attack in September 2012 the military commanders did what they could under the time and distance constraints to respond. Those investigations did not find any evidence that there was a so-called "stand down" order given by someone in the government or military that prevented any U.S. forces from getting to the site to help save anyone under attack.
Cummings and Smith highlight the Benghazi committee's counsel's exchange with Panetta's former Chief of Staff, Jeremy Bash, in January. "I would posit that from my perspective, having looked at all the materials over the last 18 months, we could not have affected the response to what occurred by 5:15 in the morning on the 12th of September in Benghazi, Libya," Chipman said.
But Wolking maintains in his statement to CNN that the Pentagon "admitted the map it previously provided to the committee showing the forces available on the night of the attacks was incomplete. Democrats' false attacks on legitimate congressional oversight are proof they're nervous about the new information committee investigators have uncovered."
In addition to the GOP Chairman's criticism of the Pentagon, the panel has also clashed for months with the State Department over its slow progress turning over documents.
Gowdy and other Republican members on the panel have told CNN they believe they are in the final stages of the investigation and hope to release a final report by this summer.