The U.S. Federal Bureau of Investigation and the Securities and Exchange Commission are investigating possible insider trading involving billionaire investor Carl Icahn, golfer Phil Mickelson and Las Vegas gambler William Walters, a source familiar with the matter said. Federal investigators are looking into whether Mickelson and Walters may have traded illegally on private information provided by Icahn about his investments in public corporations, the source told Reuters, confirming reports on Friday.