On the night of Sept. 11, as the Obama administration scrambled to respond to the Benghazi terror attacks, then-Secretary of State Hillary Clinton and a key aide effectively tried to cut the department's own counterterrorism bureau out of the chain of reporting and decision-making, according to a "whistle-blower" witness from that bureau who will soon testify to the charge before Congress, Fox News has learned.

That witness is Mark I. Thompson, a former Marine and now the deputy coordinator for operations in the agency’s counterterrorism bureau. Sources tell Fox News Thompson will level the allegation against Clinton during testimony on Wednesday before the House Oversight and Government Reform Committee, chaired by Rep. Darrell Issa, R-Calif.

Fox News has also learned that another official from the counterterrorism bureau -- independently of Thompson -- voiced the same complaint about Clinton and Under Secretary for Management Patrick Kennedy to trusted national security colleagues back in October.

Extremists linked to Al Qaeda stormed the U.S. Consulate and a nearby annex on Sept. 11, in a heavily armed and well-coordinated eight-hour assault that killed the U.S. ambassador to Libya, Chris Stevens, and three other Americans.

Thompson considers himself a whistle-blower whose account was suppressed by the official investigative panel that Clinton convened to review the episode, the Accountability Review Board (ARB). Thompson's lawyer, Joseph diGenova, a former U.S. attorney, has further alleged that his client has been subjected to threats and intimidation by as-yet-unnamed superiors at State, in advance of his cooperation with Congress.

Sources close to the congressional investigation who have been briefed on what Thompson will testify tell Fox News the veteran counterterrorism official concluded on Sept. 11 that Clinton and Kennedy tried to cut the counterterrorism bureau out of the loop as they and other Obama administration officials weighed how to respond to -- and characterize -- the Benghazi attacks.

"You should have seen what (Clinton) tried to do to us that night," the second official in State's counterterrorism bureau told colleagues back in October. Those comments would appear to be corroborated by Thompson's forthcoming testimony.

State Department spokeswoman Jen Psaki called the counterterrorism officials' allegation "100 percent false." A spokesman for Clinton said tersely that the charge is not true.

Daniel Benjamin, who ran the department's Counterterrorism Bureau at the time, also put out a statement Monday morning strongly denying the charges.

"I ran the bureau then, and I can say now with certainty, as the former Coordinator for Counterterrorism, that this charge is simply untrue," he said. "Though I was out of the country on official travel at the time of the attack, I was in frequent contact with the Department. At no time did I feel that the Bureau was in any way being left out of deliberations that it should have been part of."

He went on to call his bureau a "central participant in the interagency discussion about the longer-term response to Benghazi." He said "at no time was the Bureau sidelined or otherwise kept from carrying out its tasks."

Thompson's attorney, diGenova, would not comment for this article.

Documents from the State Department, the Central Intelligence Agency and the National Security Council, first published in the May 13 edition of "The Weekly Standard," showed that senior officials from those agencies decided within days of the attacks to delete all references to Al Qaeda's known involvement in them from "talking points" being prepared for those administration officers being sent out to discuss the attacks publicly.

Those talking points -- and indeed, the statements of all senior Obama administration officials who commented publicly on Benghazi during the early days after the attacks -- sought instead to depict the Americans' deaths as the result of a spontaneous protest that went awry. The administration later acknowledged that there had been no such protest, as evidence mounted that Al Qaeda-linked terrorists had participated in the attacks. The latter conclusion had figured prominently in the earliest CIA drafts of the talking points, but was stricken by an ad hoc group of senior officials controlling the drafting process. Among those involved in prodding the deletions, the documents published by "The Weekly Standard" show, was State Department spokeswoman Victoria Nuland, who wrote at one point that the revisions were not sufficient to satisfy "my building's leadership."

The allegations of the two counterterrorism officials stand to return the former secretary of state to the center of the Benghazi story. Widely regarded as a leading potential candidate for the Democratic presidential nomination in 2016, Clinton has insisted she was not privy to decisions made by underlings about the inadequate security for the U.S. installations in Benghazi that were made in the run-up to the attacks. And she has portrayed her role -- once the attacks became known in Washington -- as that of a determined fact-finder who worked with colleagues to fashion the best possible response to the crisis.

Clinton testified about Benghazi for the first and only time in January of this year, shortly before leaving office. She had long delayed her testimony, at first because she cited the need for the ARB to complete its report, and then because she suffered a series of untimely health problems that included a stomach virus, a concussion sustained during a fall at home, and a blood clot near her brain, from which she has since recovered. However, Clinton was never interviewed by the ARB she convened.

Fox News disclosed last week that the conduct of the ARB is itself now under review by the State Department's Office of Inspector General. A department spokesman said the OIG probe is examining all prior ARBs, not just the one established after Benghazi.

The two U.S. officials -- former Joint Chiefs of Staff Chairman Mike Mullen and former Ambassador Tom Pickering -- who oversaw the internal review of the attacks defended their report.

"From the beginning of the ARB process, we had unfettered access to everyone and everything including all the documentation we needed. Our marching orders were to get to the bottom of what happened, and that's what we did," they said in a statement Monday.

The counterterrorism officials, however, concluded that Clinton and Kennedy were immediately wary of the attacks being portrayed as acts of terrorism, and accordingly worked to prevent the counterterrorism bureau from having a role in the department's early decision-making relating to them.

Also appearing before the oversight committee on Wednesday will be Gregory N. Hicks, the deputy chief of mission at the U.S. Embassy in Libya at the time of the Benghazi terrorist attacks. Like Thompson, Hicks is a career State Department official who considers himself a Benghazi whistle-blower. His attorney, Victoria Toensing, a former chief counsel to the Senate Intelligence Committee, has charged that Hicks, too, has faced threats of reprisal from unnamed superiors at State. (Toensing and diGenova, who are representing their respective clients pro bono, are married.)

Portions of the forthcoming testimony of Hicks -- who was one of the last people to speak to Stevens, and who upon the ambassador's death became the senior U.S. diplomat in Libya -- were made public by Rep. Issa during an appearance on the CBS News program "Face the Nation" on Sunday.

Hicks told the committee that he and his colleagues on the ground in Libya that night knew instantly that Benghazi was a terrorist attack, and that he was astonished that no one drafting the administration's talking points consulted with him before finalizing them, or before U.N. Ambassador Susan Rice delivered them on the Sunday talk shows of Sept. 16.

Hicks was interviewed by the ARB but Thompson was not, sources close to the committee's investigation tell Fox News.