(FORTUNE Magazine) – Former AIG CEO Maurice "Hank" Greenberg is already facing off against Eliot Spitzer, the SEC, and the U.S. Department of Justice. Now he's increasingly at odds with another powerful figure: Warren Buffett, Berkshire Hathaway's CEO. Both men are scheduled to answer investigators' questions about suspect insurance deals (Buffett in an interview on April 11, Greenberg in a deposition on April 12). Expect Greenberg to be grilled about a deal AIG did with Berkshire's General Re unit, which may have enabled AIG to improperly boost its reserves. He'll also be asked tough questions about deals AIG struck with several offshore insurers that appear to have worked similar financial wonders. It's possible Greenberg may refuse to talk. On the other hand, Buffett is considered a cooperating witness, and inquisitors plan to restrict their questions to the AIG--General Re deal. It may be a short conversation. While Greenberg was personally involved, Berkshire Hathaway insists Buffett wasn't briefed on its structure or purpose. That said, investigators say Berkshire went out of its way to help them understand the General Re--AIG deal. As a result the probe heated up, and Greenberg is now fending off federal prosecutors. Buffett may have legions of admirers, but we're guessing Greenberg is no longer among them. -- Devin Leonard