The investigation into possible insider trading between billionaire Carl Icahn, pro-golfer Phil Mickelson and Las Vegas gambler William Walters is very serious, but will probably produce only "smoke, not fire," a former federal judge told CNBC on Monday.

Though Richard Holwell, a partner at Holwell Shuster & Goldberg, is not involved with or privy to the details of the investigation, he does know a thing or two about insider trading cases. After all, he served as a federal judge in the Southern District of New York from 2003 to 2012.