Their identities have been a well-guarded secret, known only to their high-powered lawyers and a handful of House lawmakers and staff. But now Fox News has learned the names of the self-described Benghazi “whistle-blowers” who are set to testify before a widely anticipated congressional hearing on Wednesday.

Appearing before the House Oversight and Government Reform Committee will be three career State Department officials: Gregory N. Hicks, the deputy chief of mission at the U.S. Embassy in Libya at the time of the Benghazi terrorist attacks; Mark I. Thompson, a former Marine and now the deputy coordinator for Operations in the agency’s Counterterrorism Bureau; and Eric Nordstrom, a diplomatic security officer who was the regional security officer in Libya, the top security officer in the country in the months leading up to the attacks.

U.S. Ambassador Christopher Stevens and three other Americans were killed in the Sept. 11, 2012, attacks on the U.S. outpost in Benghazi, Libya.

At the time of Stevens' death, Hicks became the highest-ranking American diplomat in Libya.

Nordstrom previously testified before the oversight committee, which is chaired by Rep. Darrell Issa, R-Calif., in October 2012. Of the three witnesses, he is the only one who does not consider himself a whistle-blower. At last fall's hearing, however, Nordstrom made headlines by detailing for lawmakers the series of requests that he, Ambassador Stevens, and others had made for enhanced security at the U.S. consulate in Benghazi in the period preceding the attacks, requests mostly rejected by State Department superiors.

More On This... How Bad Was Benghazi Botch?

"For me the Taliban is on the inside of the [State Department] building," Nordstrom testified, angry over inadequate staffing at a time when the threat environment in Benghazi was deteriorating,

The other two witnesses have not been heard from publicly before.

Hicks is a veteran Foreign Service officer whose overseas postings have also included Afghanistan, Syria, and Yemen.

Rep. Jason Chaffetz, a Utah Republican and committee member, said Hicks was in Tripoli at 9:40 p.m. local time when he received one of Stevens’ earliest phone calls amid the crisis.

“We’re under attack! We’re under attack!” the ambassador reportedly shouted into his cellphone at Hicks.

Chaffetz, who subsequently debriefed Hicks, also said the deputy “immediately called into Washington to trigger all the mechanisms” for an inter-agency response.

“The real-life trauma that [Hicks] went through,” Chaffetz recalled to Fox News’ Greta Van Susteren, “I mean, I really felt it in his voice. It was hard to listen to. He’s gone through a lot, but he did a great job.”

According to the State Department website, Thompson “advises senior leadership on operational counterterrorism matters, and ensures that the United States can rapidly respond to global terrorism crises.”

Five years before the Benghazi attacks, he lectured at a symposium hosted by the University of Central Florida and titled “The Global Terrorism Challenge: Answers to Key Questions.”

Joe diGenova, a former U.S. attorney, and wife Victoria Toensing, a former chief counsel to the Senate Intelligence Committee -- Republicans -- disclosed this week that in their private practice in the nation’s capital, they now represent pro bono two career State Department employees who regard themselves as “whistle-blowers” and would be testifying before Issa’s committee at its next Benghazi hearing, on May 8.

The lawyers said their clients believe their accounts of Benghazi were spurned by the Accountability Review board (ARB), the official investigative body convened by then-Secretary of State Hillary Clinton to review the terrorist attacks, and that the two employees have faced threats and intimidation from as-yet-unnamed superiors.

“I'm not talking generally, I'm talking specifically about Benghazi -- that people have been threatened,” Toensing told Fox News on Wednesday. “And not just the State Department; people have been threatened at the CIA. ... It's frightening. ...They're taking career people and making them well aware that their careers will be over.”

DiGenova told Fox News on Thursday, by way of describing his and Toensing’s respective clients: “There were people who were material witnesses, who wanted to talk to [the ARB], and they were not allowed to talk to them.

“The people that we are representing are career civil servants...people who have served the country overseas…in dangerous positions all over the world, have risked their lives and only want to tell the truth.”