On eve of hearing, FBI says it revoked security clearances of 3 agents over Jan. 6

The FBI has revoked the security clearances of three agents for issues related to the Jan. 6 attack on the U.S. Capitol, according to a letter from the bureau to congressional investigators obtained by ABC News.

One of the agents was among the pro-Trump mob at the U.S. Capitol, according to the FBI, and the two others are alleged to have hindered investigative efforts.

At least two of the agents -- Steve Friend and Marcus Allen -- are expected to testify Thursday before the Republican-led House select subcommittee on Weaponization of the Federal Government as part of its probe into the federal government's purported wrongdoings against conservatives.

The letter could lead to a contentious hearing, providing what could be fresh ammunition to Democrats as they attempt to undercut the credibility of the committee's witnesses and bolster their claims that Republicans are seeking to whitewash the seriousness of the assault on the Capitol.

Russell Dye, a spokesperson for committee Chairman Jim Jordan, R-Ohio, said the letter was an act of desperation by the FBI.

"This is a last minute Hail Mary from the FBI in a desperate attempt to salvage their reputation ... before brave whistleblowers testify about the agency's politicized behavior and retaliation against anyone who dares speak out," Dye told ABC News.

According to the letter, agent Brett Gloss knowingly entered a restricted zone around the U.S. Capitol during the Jan. 6 attack, in violation of the law.

"The FBI reviewed communications in which Mr. Gloss expressed support for the protestors' unauthorized entry into the Capitol building and support for their criminal acts against the U.S," said the letter, which was sent Wednesday.

The letter also said Gloss provided false or misleading information during an interview about what he observed at the Capitol on Jan. 6, and whether he entered the restricted zone.

The agents' security clearances were revoked earlier this month, the letter said.

"Specifically, the Security Division found Mr. Allen espoused alternative theories to coworkers verbally and in emails and instant messages sent on the FBI systems, in apparent attempts to hinder investigative activity," the letter to investigators said, alleging that Allen failed to provide relevant information about criminal activity at the Capitol on Jan. 6.

The bureau also says that Friend refused to participate in a court-authorized search and arrest of a criminal suspect and also espoused an alternate narrative of Jan. 6.

In addition to these concerns, the FBI also said in their letter that Friend would post sensitive FBI information on his social media accounts and did an interview with a Russian news agency without approval from the FBI's office of public affairs.