Associates of a Ukrainian oligarch fighting extradition to the U.S. were working to dig up dirt on former Vice President Joe Biden last summer in an effort to get Rudy Giuliani’s help in the oligarch’s legal case, according to three people familiar with the exchanges, Bloomberg News reports.

Dmitry Firtash, charged with conspiracy by the U.S. and living in Vienna, shuffled lawyers in July to add Joe diGenova and Victoria Toensing, vocal supporters of President Donald Trump who had worked with Giuliani. Around that time, some of Firtash’s associates began to use his broad network of Ukraine contacts to get damaging information on Biden, the people said.