Democrats on the House Select Committee on Benghazi are once again accusing their Republican counterparts of piling "repeated, unnecessary and ever-changing demands" on the Pentagon as their two-year congressional probe winds down.

In a letter to Chairman Trey Gowdy sent Monday, Democratic Reps. Elijah Cummings and Adam Smith cited testimony from retired Gen. Dana Chipman, the majority's former chief counsel, that suggested he agreed with the premise that nearby military forces could not have reached Benghazi in time to prevent the deaths of four Americans.

"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 in a January interview with Jeremy Bash, the Defense Department's former chief of staff.

Bash was the author of an email that indicated military forces were "spinning" and ready for deployment in the early hours of the Benghazi attack, a claim committee Democrats have repeatedly attempted to dismiss.

The sharply-worded letter from panel Democrats came days after an Air Force whistleblower argued jets stationed in Italy could have flown to Libya before the violence was over, potentially affecting the outcome of the attack.

After Republicans said they planned to locate and question the anonymous whistleblower, the committee minority repeated its assertion that none of the Defense Department witnesses interviewed so far have provided evidence to contradict the administration's claim that nothing could have been done to rescue the Benghazi victims.

But panel Republicans circulated a statement from Chipman on Monday that indicated the former chief counsel actually agreed with Gowdy's pursuit of the whistleblower, contrary to the suggestions in the letter from Cummings and Smith.

"If some witnesses refer the committee to other witnesses, the responsible thing to do is interview them," Chipman said. "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."

Matt Wolking, a spokesman for committee Republicans, said the Democrats have discouraged interviews with military witnesses due to "politically motivated, predetermined conclusions" about the Benghazi attack.

"No matter how many dishonest letters Democrats waste time writing, Republicans will continue conducting a thorough, fact-centered investigation that includes all relevant witnesses, regardless of rank," Wolking said.

The latest public dust-up between committee Republicans and Democrats began late last month when a Pentagon political appointee lamented the burdensome pace of congressional investigators' requests for interviews and documents.

Gowdy quickly countered that the Defense Department has readied far more witnesses in shorter time periods for other investigations, pointing to the Pentagon's objections as the latest example of the Obama administration's attempts to stonewall his inquiry.

"Democrats' false attacks on legitimate congressional oversight are proof they're nervous about the new information committee investigators have uncovered," Wolking said Monday.

The Benghazi committee is expected to publish its highly-anticipated report next month.