Carl Icahn had a bad day Monday, losing an estimated $420 million according to a report in Forbes.

Carl Icahn

The loss stems from a 4% drop in the stock of Icahn Enterprises LP (NASDAQ:IEP) that was based on revelations in the Wall Street Journal on Friday that Carl Icahn may be at the center of an insider trader case involving golfer Phil Mickelson and infamous professional gambler Billy Walters.

Golfer approached by FBI after round at Memorial golf tournament

The Journal reported that Thursday Mickelson, fresh of finishing a round of golf in a PGA tour event in Columbus, Ohio, was approached by FBI agents after he handed in his score card “I have done absolutely nothing wrong,” Mickelson declared at the Memorial golf tournament. “That’s why I’ve been fully cooperating with the FBI agents, and I’m happy to do so in the future, too, until this gets resolved.”

The allegation is that in 2011 Carl Icahn told Walters about his forthcoming investment in Clorox before it was public.  Walters is alleged to have told Mickelson, who is known to be deeply involved in gambling.  This all occurred before Carl Icahn’s intentions were public, at which point the stock of Clorox rose 7.6 percent immediately following Carl Icahn’s public announcement.

Is Carl Icahn’s action illegal?

Did Carl Icahn break the law? The New York Times is speculating that even if Icahn did provide information to Walters it might have only violated a fiduciary responsibility to investors in his company, not violated the law. Carl Icahn is 88% owner of Icahn Enterprises LP (NASDAQ:IEP).

For his part, Carl Icahn remains defiant, pointing the finger at the Wall Street Journal for reporting about the incident. “We believe that making inflammatory and speculative statements, especially when we’ve had an unblemished record for 50 years, is completely irresponsible on the part of the Wall Street Journal,” he told Bloomberg.

Think the $420 million one day hit Icahn experienced is bad? His stock lost nearly $830 million in value after the Federal Trade Commission announced an investigation into nutrition company Herbalife Ltd. (NYSE:HLF), highlighting a rare defeat in a rather public battle with fellow activist hedge fund manager Bill Ackman, who had lobbied regulators hard for action against Herbalife.

What’s a few hundred million here and there?  As Forbes notes, with an estimated worth of $23.6 billion, Carl Icahn is still the most wealthy trader on Wall Street.