EXCLUSIVE: The FBI maintained more than 40 confidential human sources on various criminal matters related to the Biden family, including Joe Biden, dating back to his time as vice president, according to information obtained by Sen. Chuck Grassley, R-Iowa.

The confidential human sources "provided criminal information to the FBI relating to Joe Biden, James Biden, and Hunter Biden." Those confidential human sources were managed by multiple FBI field offices across the nation, including the FBI’s Seattle Field Office.

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But Grassley learned that an FBI task force within the Washington Field Office sought to, and in some cases, successfully, shut down reporting and information from those sources by falsely discrediting the information as foreign disinformation. That effort "caused investigative activity to cease."

Joe and Hunter Biden

President Biden and son Hunter Biden. (AP Photo/Andrew Harnik)

However, despite those efforts by the FBI task force, Grassley said in at least one instance, a confidential human source and its information had been vetted by multiple U.S. attorneys' offices, which found "no hits to known sources of Russian disinformation."

The revelations were laid out in a letter Grassley wrote to Attorney General Merrick Garland and FBI Director Christopher Wray late Tuesday night. The letter was exclusively obtained by Fox News Digital.

"Based on the information provided to my office over a period of years by multiple credible whistleblowers, there appears to be an effort within the Justice Department and FBI to shut down investigative activity relating to the Biden family," Grassley wrote to Garland and Wray. "Such decisions point to significant political bias infecting the decision-making of not only the Attorney General and FBI Director, but also line agents and prosecutors."

He added: "Our Republic cannot survive such a political infection and you have an obligation to this country to clear the air."

James Biden

James Biden, brother of President Biden. (AP)

Grassley has been investigating for years information, records, and allegations from multiple Justice Department whistleblowers that indicate "there is — and has been — an effort among certain Justice Department and FBI officials to improperly delay and stop full and complete investigative activity into the Biden family, including but not limited to FD-1023s referencing the Biden family."

An FD-1023 is an FBI-generated form used to document confidential human source reporting.

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"An essential question that must be answered is this: did the FBI investigate the information or shut it down?" Grassley wrote, noting that if those sources were "improperly shut down, it wouldn’t be out of the ordinary for the FBI."

One critical FD-1023 in question was first reported on by Fox News Digital earlier this year. That form included reporting from a "highly-credible" confidential human source who alleged a criminal bribery scheme between then-Vice President Joe Biden, his son Hunter Biden, and the founder and CEO of Ukrainian natural gas firm Burisma Holdings, Mykola Zlochevsky.

Ukrainian businessman and Burisma Holdings company founder Mykola Zlochevsky.

Ukrainian businessman and Burisma Holdings company founder Mykola Zlochevsky. (Getty Images)

The form details multiple meetings and conversations the source had with a top executive of Burisma Holdings over the course of several years starting in 2015. Hunter Biden, at the time, sat on the board of Burisma.

The source told the FBI that Joe Biden and Hunter Biden allegedly "coerced" Zlochevsky to pay them millions of dollars in exchange for their help in getting the Ukrainian prosecutor investigating the company, Viktor Shokin, fired.

The White House has denied the allegations.

But that form and those allegations are "part of an ongoing investigative matter," which Grassley says indicates "its investigative credibility and authenticity."

In the letter transmitted to the DOJ and FBI late Tuesday, Grassley revealed that in December 2019, the FBI Washington Field Office closed a "205B" Kleptocracy case into Zlochevsky. That probe had been opened in January 2016 by a Foreign Corrupt Practices Act squad based out of that same field office — a squad that included agents from FBI headquarters.

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At the time of the closing of the probe, in December 2019, Hunter Biden’s role on the board of Burisma was heavily under the microscope amid the first impeachment of former President Donald Trump. The impeachment proceedings were based on a request from Trump to Ukrainian President Volodymyr Zelenskyy to investigate the Biden family’s business dealings in Ukraine and why the prosecutor investigating Burisma had been fired.

Sen. Chuck Grassley speaks into mircrophone during hearing

Sen. Chuck Grassley, R-Iowa. (Al Drago/Bloomberg via Getty Images)

Months later, in February 2020, Grassley said a meeting took place at the FBI’s Pittsburgh Field Office, which involved discussion about investigative matters relating to the Hunter Biden investigation and related inquiries. By March 2020, a "guardian" assessment was opened in that office to analyze information about the Bidens provided by then-Trump attorney Rudy Giuliani.

During the course of that assessment, DOJ and FBI officials located an FD-1023 from March 1, 2017, relating to the kleptocracy investigation of Zlochevsky. That document included a reference to Hunter Biden being on the board of Burisma, which "the handling agent deemed at the time non-relevant information to the ongoing criminal financial case." 

"When that FD-1023 was discovered, Justice Department and FBI officials asked the handler for the Confidential Human Source to re-interview that CHS," Grassley said.

Christopher Wray

FBI Director Christopher Wray. (AP Photo/Carolyn Kaster)

Eventually, that re-interview took place, Grassley said, and the aforementioned FD-1023 alleging a criminal scheme between then-Vice President Biden, Hunter Biden, and Zlochevsky was created in June 2020.

But federal prosecutors in Pittsburgh were "limited in their investigative abilities." Grassley said, because it was merely an "assessment." Prosecutors were not able to issue subpoenas and search warrants, but instead, only able to conduct "database checks."

"My office has been informed that the FBI agents and DOJ officials working the Pittsburgh Assessment had to pause their work for weeks at a time because the assessment had to be re-approved every 30 days by multiple DOJ and FBI officials," Grassley wrote in the letter.

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Several months later, in August 2020, FBI Supervisory Intelligence Analyst Brian Auten opened an assessment, which was used by the Foreign Influence Task Force, to seek out confidential human source information at FBI field offices across the country relating to the Biden family. Grassley said he then worked to "falsely discredit them as foreign disinformation."

The task force "attempted to shut down" the investigative steps on the FD-1023 in question, saying it "was subject to foreign disinformation." 

"It should be emphasized that the basis for trying to shut down the Biden family 1023 has been described to my office as highly suspect and is contradicted by other documents my office has been told exist within the Foreign Influence Task Force, FBI Seattle Field Office, FBI Baltimore Field Office, and FBI HQ holdings," Grassley wrote.

Merrick Garland

Attorney General Merrick Garland. (Chip Somodevilla/Getty Images)

During that same time period, in August 2020, that task force traveled to Capitol Hill and "improperly briefed" Grassley and Johnson on their investigation into the Biden family after "pressure from congressional Democrats."

"That improper briefing was used by Democrats and the partisan media to falsely claim our investigation, which was based on Obama/Biden administration records, was impacted by foreign disinformation," Grassley wrote.

The Pittsburgh assessment was ultimately closed weeks later, in September 2020, and a final report of its findings was transmitted by U.S. Attorney for the Western District of Pennsylvania, Scott Brady to the main Justice Department — specifically directed to Principal Associate Deputy Attorney General Rich Donoghue.

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The subject line of that report, dated Sept. 21, 2020, was: "VETTING."

In that report, Brady noted that investigators "had limited ability to verify all information," due to their lack of authorization for a grand jury. Investigators were unable to issue subpoenas for relevant documents or for interviews. The report, though, included a recommendation that "additional investigative activity be done." 

Brady’s office, did, however, coordinate with the U.S. Attorney’s Office for the Eastern District of New York in 2020 on the Biden FD-1023. Both U.S. attorneys' offices "found no hits to known sources of Russian disinformation" related to the FD-1023 in question.

The report was eventually transmitted to now-Special Counsel David Weiss, who has been investigating Hunter Biden since 2018. That probe was launched initially as a money laundering investigation and a Foreign Agents Registration Act (FARA) case. That investigation is ongoing.

Meanwhile, on a call in early October 2020, it became clear that FBI agents in Delaware "were in possession of email evidence that contradicted denials made by Joe Biden that he was never aware of or involved in Hunter Biden’s business arrangements."

Hunter Biden gets off plane with president

President Biden and his son, Hunter Biden. (AP Photo/Patrick Semansky)

Assistant U.S. Attorney Lesley Wolf was briefed in late October 2020 on the contents of the FD-1023 in question, but Grassley said she "prevented investigators from seeking information about Joe Biden’s involvement in Hunter Biden’s criminal arrangements." IRS whistleblowers involved in the investigation have also made that allegation.

Grassley also noted that in October 2020, an "avenue of derogatory Hunter Biden reporting was ordered closed" at the direction of Assistant Special Agent in Charge Tim Thibault — a move that whistleblowers said was made "in furtherance of Auten’s assessment."

Grassley said publicly released portions of Thibault’s interview with the House Judiciary Committee confirmed those allegations and reveals that an FBI headquarters and "Baltimore element wanted the Hunter Biden reporting closed which Thibault followed through on." 

The FBI declined to comment on Grassley's letter. The Justice Department did not immediately respond to Fox News Digital's request for comment. 

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Weeks later, after the 2020 presidential election, Grassley and Johnson made public their final report out of their probe, detailing their findings based on hundreds of Treasury Department Suspicious Activity Reports, interviews and thousands of pages of government records dating back to the Obama administration.

Special Counsel David Weiss

U.S. Attorney for the District of Delaware David C. Weiss. (Fox News screenshot)

"Our findings indicated potential criminal activity, to include money laundering, with respect to members of the Biden family and their business associates as well as strong financial connections to questionable foreign nationals and foreign government-linked corporate entities," Grassley wrote.

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Since, Grassley has been in touch with Weiss’ team, and has provided hundreds of pages of bank records "connecting the Biden family to Chinese-government linked entities."

Grassley demanded the FBI and DOJ provide answers to his letter by Nov. 17.