As the House kicks off public impeachment hearings this week, a trio of Democratic senators are refocusing scrutiny on an important player in the Trump administration’s Ukraine drama: Attorney General William Barr.

On Tuesday, the Democrats—Sens. Kamala Harris (D-CA), Sheldon Whitehouse (D-RI), and Richard Blumenthal (D-CT)—sent a letter to the Department of Justice’s Inspector General, Michael Horowitz, asking him to investigate whether Barr was pushed by President Trump or anyone in the White House to open investigations into U.S. political figures like the Bidens.

The request has its roots in Barr’s appearance before the Senate Judiciary Committee in May, when Barr proved unable to answer a question from Harris about whether Trump or another White House official ever directly asked or suggested that he investigate anyone. The exchange, which came during Barr’s first testimony following the release of the Mueller report, spread on liberal corners of social media due to Barr’s non-committal answers.

“I don’t know,” Barr ultimately said, after parsing the wording of Harris’ question.

After the hearing, Harris wrote a letter to Horowitz, asking him to investigate. In the Tuesday letter, the senators write that Horowitz never responded or confirmed whether or not an investigation is occurring.

The senators say that Barr’s role in the apparent Trumpworld campaign to pressure the Ukrainian government to investigate his political rivals renews the urgency of their request. In Trump’s July 25 phone call with Volodymyr Zelensky, he urged his Ukrainian counterpart to have his staff work with Barr to investigate the Biden family, their business interests in the country, and any role the former vice president played in the firing of a Ukrainian prosecutor. Trump also tells Zelensky that he plans to direct Barr and his personal lawyer, Rudy Giuliani, to follow up with Zelensky to discuss the issue further.

On April 25, when the Ukraine scandal was beginning to brew behind the scenes, Trump told Fox News’ Sean Hannity that Barr was looking into evidence circulating in conservative media that the Ukrainians interfered to hurt Trump in the 2016 election. “I would imagine [Barr] would want to see this,” Trump said.

The senators write that “It strains credulity to maintain that Attorney General Barr was unaware of the president’s desire for him to conduct such investigations—investigations that the president mentioned to a foreign head of state and spoke of openly on cable television.”

“These episodes again raise questions about whether the Department of Justice and its leadership participate in politically motivated investigations—a practice that threatens not only the independence of the Justice Department, but of the rule of law,” write Harris, Whitehouse, and Blumenthal. “These disturbing revelations, as well as the attorney general’s unwillingness or inability to be forthcoming when asked under oath about the president or White House’s involvement in Justice Department investigations, warrant the prompt attention of your office.”

The Justice Department did not immediately respond to a request for comment on the matter from The Daily Beast.

Horowitz is expected to testify in front of the Senate Judiciary Committee as soon as this month. The senators indicated that if he does not respond to their letter, they will press him on the matter in person.

Barr, meanwhile, has remained in the background of the impeachment probe, as House Democrats zero in on State Department and National Security Council officials who have been willing to testify about the administration’s interactions with Ukraine.