Oleg Deripaska, a close ally of Russian President Vladimir Putin and an oligarch who has been linked in reports to President Trump’s former campaign manager, has reportedly offered to testify in the ongoing probe into Russia’s election interference. Lawmakers declined the offer, however, saying Deripaska’s demand for full immunity was not acceptable, three congressional officials told The New York Times late Friday. Deripaska’s name has repeatedly come up in the Russia investigation for his ties to Paul Manafort, Trump’s former campaign manager. The Associated Press reported in March that Manafort plotted with Deripaska to influence U.S. foreign policy to “greatly benefit the Putin government” in 2005, a claim which Deripaska and Manafort both have denied. While Deripaska and Manafort did business together in the mid-2000s, their relationship has since gone south after Deripaska accused Manafort of misusing funds for a Cayman Islands deal. Manafort stepped down from Trump’s campaign amid increasing scrutiny over his Russia ties last August, though his lobbying activities for Russia continue to be a main focus of the FBI’s probe into ties between Trump’s campaign and Moscow.