The top Republican on the Senate Judiciary Committee, Chuck Grassley, R-Iowa, is pressing the Department of Justice and the FBI for an explanation about why they pursued "politically charged investigations" related to the Trump campaign while suppressing key details related to the Hunter Biden probe as "disinformation," in light of new whistleblower information.

In a letter to Attorney General Merrick Garland and FBI Director Chris Wray sent on July 18 and first reviewed by Fox News Digital, Grassley raises concerns about a "pattern of active public partisanship" by employee Assistant Special Agent in Charge Timothy Thibault, and others.

In an additional letter to Garland and Wray sent Monday, Grassley narrows in on specific allegations about Thibault which he says could have improperly influenced the criminal investigation into President Biden's son, Hunter.

Grassley's new round of letters follow a May 31 request for an Inspector General investigation into Thibault due to his alleged partisan content on social media, including re-posting tweets by the anti-Trump group the Lincoln Project and other accounts attacking Republicans. The senator writes this week that Thibault has since hidden his social media posts from public view.

HUNTER BIDEN MAKES RARE PUBLIC OUTING AMID INVESTIGATION

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Grassley sent two new letters to the FBI and DOJ regarding the issues of partisan politics at play in the government's conduct of the Hunter Biden investigation. (Fox News)

In addition to Grassley's outreach in May to the agencies, his office says it has been contacted by current and former "highly credible whistleblowers" who have provided information that he says "confirms" his concerns about Thibault's pattern of partisanship.

"In fact, the information I have received reveals that Thibault's political partisanship went much deeper than the inappropriate social media posts. Instead, it impacted his official decision-making on sensitive public corruption investigations," writes Grassley. "Equally concerning is that, based on Justice Department and FBI policies, Thibault's partisanship likely affected investigations briefed to, and approved by, senior Justice Department and FBI officials."

According to Grassley, the whistleblowers say that DOJ and FBI employees must follow strict substantial factual predication in order to open an investigation, and that Thibault did not follow these policies. 

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"The double standard in the application of Justice Department and FBI policies has resulted in investigations opened in a manner appearing to benefit the political aims and objectives of a select few Justice Department and FBI officials. Further, it is my understanding from whistleblowers that when FBI agents sought a uniform and consistent application of these standards, their concerns were marginalized or otherwise not acted upon," Grassley wrote.

Hunter Biden in Malibu, California

Biden administration officials and those in the president’s camp have long cast materials that came from the laptop purportedly belonging to Hunter Biden as disinformation, but intelligence community officials have said, as far back as October 2020, that neither the laptop nor materials on it are part of any sort of disinformation campaign. (RMBI / BACKGRID)

In addition, Grassley says the whistleblowers indicated that Thibault and Richard Pilger, who serves in the election crimes branch of DOJ's Public Integrity Section, were at the center of the ongoing investigation into former President Trump's campaign regarding 2020 electors.

The senator from Iowa states that the investigation predication that was presented to the attorney general and FBI director for final sign-off for full investigation was based on news articles based on information collected from American Oversight, a "liberal" government watchdog.

"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 and FBI's usual process and procedure to open and pursue high-profile and politically charged investigations," Grassley writes, demanding further explanation from the agencies.

On Hunter Biden, Grassley writes that the new whistleblower information provided to his office "involves concerns about the FBI's receipt and use of derogatory information related to Hunter Biden, and the FBI's false portrayal of acquired evidence as disinformation."

Grassley says the whistleblower allegations indicate there was a "scheme" in place among some FBI officials to "undermine derogatory information connected to Hunter" by falsely flagging it as disinformation. 

He then continues on to outline a series of events between July and October 2020, during which the FBI, including Thibault and FBI Supervisory Intelligence Analyst Brian Auten, and Democratic senators gave Republicans investigating the Hunter allegations the runaround on information, falsely alleging that the GOP was advancing "foreign disinformation" regarding the now-president's son.

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Hunter Biden's ex-wife detailed how she heard about the affair between Biden and his brother's widow Hallie in her new memoir.  (REUTERS/Joshua Roberts)

Intelligence community officials have said, as far back as October 2020, that neither the Hunter Biden laptop nor materials on it are part of any sort of disinformation campaign.

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The federal investigation into the first son's tax affairs has reached a "critical stage," a source told Fox News last week. Officials are looking into whether to charge President Biden's son with various tax violations, possible foreign lobbying violations and more.

A separate source told Fox News that the federal grand jury looking into Hunter Biden’s business dealings wrapped up its latest term late last month but said no charges have been filed.

DOJ declined to comment on Grassley's letters. The FBI confirmed receipt of the letters but declined to comment further.

Fox News's Brooke Singman, David Spunt and Bill Mears contributed to this article.