“Highly credible” whistleblowers have come forward to a senior Senate Republican alleging a widespread effort within the FBI to downplay or discredit negative information about President Joe Biden’s son, Hunter Biden, according to letters reviewed by CBS News.

“The information provided to my office involves concerns about the FBI’s receipt and use of derogatory information relating to Hunter Biden, and the FBI’s false portrayal of acquired evidence as disinformation,” GOP Sen. Chuck Grassley wrote FBI Director Christopher Wray and Attorney General Merrick Garland on July 25. “The volume and consistency of these allegations substantiate their credibility and necessitate this letter.”

Grassley, the ranking member on the Senate Judiciary Committee, said the whistleblowers alleged that legitimate streams of information and intelligence about the president’s son were characterized as likely disinformation or prematurely shut down leading up to the 2020 presidential election.

CBS News first reported in April that more than 150 financial transactions involving either Hunter or James Biden’s global business affairs were flagged as concerning by U.S. banks for further review. CBS News previously reported the U.S. attorney in Delaware has been investigating potential tax fraud, money laundering and foreign influence peddling.

President Biden has said he has confidence his family did the right thing, and he was not involved in the business dealings of his son and brother.

A spokesperson for Delaware U.S. attorney David Weiss declined to comment on whether he was aware of whistleblower allegations that credible reporting about Hunter Biden was allegedly labeled misinformation by the FBI and whether these allegations had been factored into his probe.

In the correspondence, Grassley cited a 2020 FBI intelligence assessment that was “used by an FBI headquarters team to improperly discredit negative Hunter Biden information as disinformation.” He continued, “Based on allegations, verified and verifiable derogatory information on Hunter Biden was falsely labeled as disinformation.”

In October 2020, one month before the election, “an avenue of derogatory Hunter Biden reporting was ordered closed” by a senior FBI agent at the bureau’s Washington Field office. An earlier letter from Grassley identified the agent as Timothy Thibault.

“[T]he allegations provided to my office appear to indicate that there was a scheme in place among certain FBI officials to undermine derogatory information connected to Hunter Biden by falsely suggesting it was disinformation,” Grassley claimed.

He said whistleblowers from federal law enforcement came forward to his office after a May 31 letter he had written noted a “pattern of active public partisanship in his then public social media content,” in reference to Thibault.

In that May 31 letter, Grassley also accused Thibault of likely violations of “[f]ederal laws, regulations and Federal Bureau of Investigation (FBI) guidelines.”

“Whistleblowers have reported to me, that although the FBI and Justice Department maintain policies dictating specific standards requiring substantial factual predication to initiate an investigation, Thibault and other Justice Department and FBI employees failed to comply with these requirements.”

The letter also singled out Justice Department official Richard Pilger, identified as the director of the Election Crimes Branch within the Justice Department’s Public Integrity Section, who, along with Thibault, were alleged to be “deeply involved in the decisions to open and pursue this investigation,” an apparent reference to a probe recently opened into the Trump campaign.

Grassley told Wray and Garland “the opening memo you approved,” included media reporting citing research from a “liberal non-profit” when a full investigation of a political nature requires “a heightened factual basis.”

Grassley continued, “In light of these allegations, I remain very concerned that political bias by a select group of Justice Department and FBI officials has infected the Justice Department’s and FBI’s usual process and procedure to open and pursue high-profile and politically charged investigations.”

In addition to requesting records from the FBI and the Justice Department by Aug. 8, Grassley complained about the way the bureau and department are being run.

“Attorney General Garland and Director Wray, simply put, based on the allegations that I’ve received from numerous whistleblowers, you have systemic and existential problems within your agencies,” Grassley wrote. “You have an obligation to the country to take these allegations seriously, immediately investigate.”

The FBI said it had received the letters but declined any further comment.

Asked for comment on the letter, Justice Department said it had received the letter, declined to comment further, and did not refer CBS News to any counsel for Pilger.