The FBI has hit a roadblock in its insider-trading probe against billionaire activist-investor Carl Icahn, sports bettor William "Billy" Walters and champion golfer Phil Mickelson, as public knowledge of the investigation could complicate the bureau's efforts to eavesdrop on private conversations, a method that has helped crack similar cases in the past, according to the Wall Street Journal.

U.S. authorities are investigating if Icahn gave away information about his investment plans for several stocks and companies to Walters and Mickelson, and whether they made trading decisions based on that knowledge. The investigation, which was reported first by the Journal on Friday, began in 2011 and focuses on trading in the stock of The Clorox Co., (NYSE:CLX), an Oakland, Calif.-based laundry products company. Icahn, Walters and Mickelson have denied any wrongdoing. 

While the report on the probe, which had so far included the use of several electronic and manual devices, foils investigators' plans to secretly deploy listening devices, investigators also discovered that it would not be possible to listen in on Icahn's calls without his knowledge because he owns a stake in the telecom company with whose help the calls would be tapped, the Journal reported.

Mickelson, whose involvement in Dean Foods, which is suspected to have begun after a tip from Walters, is also being investigated by FBI agents. The golfer, who has three Masters titles and a PGA Championship to his name as well as numerous other wins, has denied knowing Icahn and said Saturday that he has done "absolutely nothing wrong" and was "fully cooperating with the FBI agents," according to the Journal.

While Mickelson’s lawyer has said that the golfer is not a target of the investigation, FBI agents have reportedly refused to comment on his involvement. Walters, who reportedly met Icahn between 1998 and 2008, at a time when Icahn Enterprises owned a casino and a hotel in Las Vegas, has not yet commented on the investigation, according to the Journal.

While the latest case is one of the most high-profile insider-trading probes because of the people involved, investigations by the FBI and the Manhattan U.S. attorney's office in the last five years have led to more than 85 pleas and convictions, the Journal reported.