Senate Judiciary Chair Lindsey Graham has opened a probe into the conduct of Joe BidenHunter Biden, and the Ukraine gas company Burisma, nearly one month after claiming his committee had no jurisdiction over the matter.

According to news first reported in the Washington Post, Graham sent a letter to Secretary of State Mike Pompeo on Thursday seeking information about the 2016 communications of then-vice president Biden with the former Ukrainian President Petro Poroshenko.

The formal request by Graham centers on the dismissal of former Ukrainian Prosecutor General Viktor Shokin, who at one point been investigating Burisma. Shokin’s firing was a key element of an anti-corruption campaign publicly called for by Biden, numerous other EU officials, Republicans in Congress, and Ukraine’s own parliament. In the letter, Graham asked the State Dept. to provide him with “call transcripts or summaries” as well as “documents and communication” between Biden and Poroshenko. Graham also suggested in his letter that because Hunter Biden began following Deputy Secretary of State Tony Blinken on Twitter in February 2016, the the two “may have initiated conversations regarding Prosecutor General Shokin’s investigation into Burisma.”