Senate Judiciary Committee Chairman Lindsey Graham (R-South Carolina) launched an investigation on Thursday into former Vice President Joe Biden‘s dealings in Ukraine, a fundamental piece in the current impeachment proceedings against President Donald Trump.

Graham sent a letter to Secretary of State Mike Pompeo requesting documents related to Biden’s communications with Ukrainian officials. He is looking into what, if any, conversations Biden had with former Ukrainian President Petro Poroshenko about the removal of the country’s top prosecutor, or about Burisma, the Ukrainian gas company where Biden’s son Hunter sat on the board.

These claims have played a central role in the Trump impeachment hearings, as the initial inquiry was sparked after a whistleblower complained that Trump asked the president of Ukraine to investigate Biden during a July 25 phone call.

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During the phone conversation, Trump asserted that Biden forced the firing of former Prosecutor General Yuriy Lutsenko, because he had been investigating Burisma on allegations of tax evasion. However, Lutsenko was widely viewed as corrupt and the Burisma investigation had been dropped by the time Biden visited Ukraine, so there is little evidence that Biden was acting in the interest of his son.

At an April meeting at the Council on Foreign Relations, Biden publicly stated that he threatened withholding a billion dollar loan guarantee from the U.S. in order to pressure Ukrainian officials to fire the state prosecutor.

In a video of the meeting, Biden recounted telling the officials, “I’m leaving in six hours. If the prosecutor’s not fired, you’re not getting the money. Well son of a bitch got fired. And they put in place someone who was solid at the time.”

Trump’s impeachment has zeroed in on a similar offense — that he withheld U.S. military aid in order to pressure Ukraine to open a political probe.

Taylor Reidy, a spokeswoman for Graham, told the Washington Post that the senator is opening the inquiry because the House Intelligence Committee leading the impeachment proceedings have neglected to look into that aspect of the Ukraine scandal.

“[Rep.] Adam Schiff and the House Intel Committee have made it clear they will not look into the issues about Hunter Biden and Burisma,” Reidy said. “Graham is requesting documents which could shed additional light on that issue and hopes they will be able to answer some of the outstanding questions.”