House Republicans are preparing to conduct the first interviews in more than four months in their investigation into the FBI’s handling of the Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE email probe.

A joint investigation run by the Judiciary and the Oversight and Government Reform committees has set three witness interviews for June, including testimony from Bill Priestap, the assistant director of the FBI’s counterintelligence division, and Michael Steinbach, the former head of the FBI’s national security division.

Multiple congressional sources confirmed Priestap’s interview. Steinbach confirmed to The Hill that he would be appearing.

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The third witness is John Giacalone, who preceded Steinbach as the bureau's top national security official and oversaw the first seven months of the Clinton probe, according to multiple congressional sources.

Priestap, in particular, has come under fire from conservatives.

As the head of the FBI counterintelligence division, he held a pivotal leadership position in both the Clinton and Russia probes and was in a supervisory position over counterintelligence agent Peter Strzok, whose text messages criticizing Trump and other political figures during the 2016 presidential race have been the focus of a maelstrom of scrutiny from the right.

Republicans are ramping back up the controversial investigation amidst what has become a direct assault by a number of conservatives on special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s investigation into the Trump campaign and Russia.

President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE's allies on Capitol Hill have alleged widespread misconduct within the FBI and the Justice Department during the 2016 election and say it is evidence of systemic bias against the president.

The allegations have spawned a number of counter-investigations and thrust a host of formerly anonymous FBI and Justice Department career officials into the limelight.

The joint Judiciary–Oversight review — led by chairs Bob Goodlatte Robert (Bob) William GoodlatteNo documents? Hoping for legalization? Be wary of Joe Biden Press: Trump's final presidential pardon: himself USCIS chief Cuccinelli blames Paul Ryan for immigration inaction MORE (R-Va.) and Trey Gowdy Harold (Trey) Watson GowdySunday shows preview: Election integrity dominates as Nov. 3 nears Tim Scott invokes Breonna Taylor, George Floyd in Trump convention speech Sunday shows preview: Republicans gear up for national convention, USPS debate continues in Washington MORE (R-S.C.), respectively — is centered on the bureau’s decisionmaking in both the investigation of Clinton’s use of a private email server while secretary of State and the investigation into possible ties between the Trump campaign and Russia.

But its progress had stalled in the months since it was first announced.

Since October, the panel is believed to have interviewed only two witnesses — of about 20 potential witnesses — infuriating conservative members who are eager to uncover what some have characterized as “corruption.”

All three interviews are scheduled separately. Priestap will appear in the first week of June, Giacalone in the second and Steinbach in the final week of the month, according to the congressional source.

Democrats have derided the probe as a partisan exercise designed to shield Trump by muddying the waters around the federal investigation into his campaign.

Gowdy has described the investigation as a serious inquiry into the bureau’s conduct during the Clinton investigation — also under the microscope of Justice Department Inspector General Michael Horowitz — rather than an effort to re-litigate the decision not to bring charges against the former secretary of State.

Horowitz is slated to release his own report imminently, raising some question about whether he will pre-empt the interviews.

The inspector general earlier this spring released a report from a completed portion of the investigation that was deeply critical of former Deputy Director Andrew McCabe Andrew George McCabeGraham: Comey to testify about FBI's Russia probe, Mueller declined invitation Barr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' GOP votes to authorize subpoenas, depositions in Obama-era probe MORE, a top target of conservatives alleging wrongdoing at the Justice Department.

The Judiciary–Oversight inquiry is running on parallel tracks to an investigation spearheaded by House Intelligence Committee Chairman Devin Nunes Devin Gerald NunesSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Sunday shows preview: With less than two months to go, race for the White House heats up Sunday shows preview: Republicans gear up for national convention, USPS debate continues in Washington MORE (R-Calif.), who has zeroed in on what he describes as surveillance abuses by the Justice Department and the FBI during the election.

But the two investigations — alongside similar probes from a handful of Senate Republicans — share many of the same targets, including Strzok.

Although lawmakers have clamored for Strzok and former FBI lawyer Lisa Page to testify, neither Strzok nor Page is expected to appear before the committee in June.

But as his subordinate, Strzok would run decisions by Priestap, his text message exchanges with Page show.

Priestap was also involved in another hotly-contested episode during the 2016 election: The decision by then-FBI Director James Comey James Brien ComeyDemocrats fear Russia interference could spoil bid to retake Senate Book: FBI sex crimes investigator helped trigger October 2016 public probe of Clinton emails Trump jabs at FBI director over testimony on Russia, antifa MORE to call Clinton’s handling of her emails “extremely careless” and not the potentially criminal “grossly negligent.”

According to records released earlier this year by Sen. Ron Johnson Ronald (Ron) Harold Johnson CIA found Putin 'probably directing' campaign against Biden: report This week: Supreme Court fight over Ginsburg's seat upends Congress's agenda GOP set to release controversial Biden report MORE (R-Wis.), Priestap reviewed and provided edits to the statement Comey gave in July 2016 announcing that he would not be recommending charges.

That statement has long fueled conservative ire. Trump and other Republicans have seized on the wording change and the revelation that Comey began drafting the statement before he had interviewed Clinton as evidence that the FBI was trying to shield Clinton from prosecution.

“Comey drafted the Crooked Hillary exoneration long before he talked to her (lied in Congress to Senator [Lindsey Graham Lindsey Olin GrahamSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE]), then based his decisions on her poll numbers. Disgruntled, he, McCabe, and the others, committed many crimes!” Trump tweeted last month.

Comey has said that he could not establish that Clinton acted with criminal intent — and that “no reasonable prosecutor” would charge Clinton based on “gross negligence,” a standard that has been used only once in the statute’s 99-year history.

Priestap’s name also appears in a controversial memo drafted by staff for Nunes, alleging that the Justice Department inappropriately obtained a surveillance warrant on Trump campaign aide Carter Page by using an opposition research dossier into Trump that was paid for in part by Clinton.

The use of allegations from the so-called Steele dossier has been ground zero for many of the conservative allegations of abuse.

According to the Nunes memo, Priestap told the committee that corroboration of the Steele dossier was in its “infancy” at the time the bureau first applied for a surveillance warrant on Page.

Steinbach, as executive assistant director of the national security division, also worked closely with Comey and Strzok on both the Clinton and Russia investigations and was involved in the drafting of the July statement.

He left the FBI last year and has been a public defender of a number of GOP targets at the FBI, including McCabe and Strzok.

“To think Pete could not do his job objectively shows no understanding of the organization," Steinbach told The Washington Post last year.

“We have Democrats, we have Republicans, we have conservatives and liberals. Having personal views doesn't prevent us from independently following the facts.”