At the time, Biden’s son Hunter held a lucrative board position with Burisma Holdings, a Ukrainian gas company. Burisma, and its owner, Mykola Zlochevsky, had faced investigative scrutiny from Shokin’s office, presenting a potential conflict of interest for Biden.

Kurt Volker, a former U.S. special envoy to Ukraine called by House Republicans to testify at impeachment hearings, called allegations of corrupt intent by Biden “self-serving and non-credible.”

A former Shokin deputy has said investigations into Burisma were long dormant by the time he secured Shokin’s firing in late March 2016. On Thursday, a key witness in the impeachment probe, David Holmes, an official at the U.S. Embassy in Kyiv, said the same during his testimony before the House Intelligence Committee.

But in February 2016, Shokin’s office seized Zlochevsky’s property as part of a corruption investigation, according to a report at the time from the news service Interfax-Ukraine.

Graham’s letter notes that Hunter Biden became a Twitter follower of Tony Blinken, a longtime Biden aide then serving as deputy secretary of State, on the day of the Interfax report and suggests the two may have discussed Shokin’s investigations. Neither the State Department, Blinken nor the Biden campaign immediately responded to a request for comment.

The letter from Graham, the chairman of the Senate Judiciary Committee, seeks all communications between Biden’s office and Poroshenko’s office between the news of the raid and Shokin’s March 29 firing.

It also seeks records related to a meeting between another Burisma board member, Devon Archer, and then-Secretary of State John Kerry on March 2, 2016.

Archer, a business partner of Hunter Biden, was also a close friend and business partner of Kerry’s stepson, Christopher Heinz.

News of the Archer-Kerry meeting, as well as Hunter Biden’s Twitter follow of Blinken were first reported by the conservative journalist John Solomon based on State Department documents he obtained via a Freedom of Information Act request.

Despite the letter and their partisan differences, Graham and Biden have a history of warm relations. Graham became visibly emotional when talking about his former Senate colleague in 2015.

“He’s the nicest person I think I’ve ever met in public life.” Graham said at the time. “He is as good a man as God ever created.”

Nahal Toosi contributed to this report.