An FBI informant has been charged with lying to authorities about a multimillion-dollar bribery scheme involving President Joe Biden, his son Hunter and a Ukrainian energy company, a claim that is central to the Republican impeachment inquiry in Congress. Alexander Smirnov falsely reported in June 2020 that executives associated with the Ukrainian energy company Burisma paid Hunter and Joe Biden $5 million each in 2015 or 2016, prosecutors said Thursday. The informant’s claims have been central to the Republican effort in Congress to investigate the president and his family, and helped spark what is now a House impeachment inquiry into Biden. Smirnov was again interviewed by FBI agents in September 2023, the indictment says, and he allegedly repeated earlier false claims and also changed his story to promote "a new false narrative after he said he met with Russian officials." Alexander Smirnov was, in many ways, the archetype of an informant operating in the shadowlands of the former Soviet Union — a profiteer, fixer and gossip who promoted his ability to make sense of a confusing landscape to American law enforcement agencies. For more than a decade, he played a double game, giving the F.B.I. tantalizing visibility into a cast of oligarchs and public officials while offering himself as a consultant, with a hard-to-define skill set, to some of the same people he was keeping tabs on. Little is known about Mr. Smirnov beyond a few public records and snippets of biography in papers filed in federal court in Las Vegas, where he lives. He appears to have no presence on social media. Mr. Smirnov, a dual citizen of Israel and the United States, moved into a three-bedroom, three-bath condo off the Las Vegas Strip that his longtime girlfriend bought two years ago for $980,000. There is no public record of prior criminal charges against him in the United States. Before that, he lived for at least 16 years in California, most recently in the affluent seaside community of Laguna Beach. It is not clear, either in court filings or public records, where Mr. Smirnov was born. He is fluent in Russian, speaks English with a heavy accent and might have roots in Ukraine, according to a person with knowledge of the situation. His lawyers cited his close personal connections with three people — his longtime girlfriend, Diana Lavrenyuk, her grown son in the District of Columbia area, and a cousin from Miami, Linor Shefer — to counter claims by prosecutors that he would flee the country if released. Ms. Shefer — who was crowned “Miss Jewish Star” in Moscow in 2014 at a pageant officiated by Israel’s ambassador to Russia,