Mickelson's manager confirmed Saturday that the investigation was the same outlined in reports in several newspapers, including the Wall Street Journal.
The newspapers' report that federal investigators are looking into the trading patterns of Mickelson and Walters involving two stocks. The reports say the FBI and Securities and Exchange Commissioner are analyzing trades Mickelson and Walters made involving Clorox about the time Icahn was attempting to take over the company.
The newspapers cited unidentified people who had been briefed on the investigation. The Associated Press could not immediately confirm details of the probe. The Justice Department and SEC declined comment.
Mickelson, one of the most popular figures in golf, was playing at the Memorial was not available for comment until after his round Saturday.
"I have done absolutely nothing wrong," Mickelson said in a statement. "I have cooperated with the government in this investigation and will continue to do so. I wish I could fully discuss this matter, but under the current circumstances it's just not possible."
The reports say the investigation is looking at Clorox trading in 2011. Icahn accumulated a 9.1 percent stake in Clorox in early 2011, and that summer made a $10.2 billion offer for the company that caused the stock value to rise. Icahn's bid for Clorox ended in September 2011.
The newspapers say Mickelson and Walters placed their Clorox trades about the same time in 2011. Federal officials are looking into whether Icahn shared information of his Clorox takeover bid with Walters, and whether Walters passed that information on to Mickelson.
The reports say investigators also are examining trades by Mickelson and Walters involving Dean Foods Co. in 2012.
Icahn, 78, is one of Wall Street's most successful corporate raiders, famous for buying stock in underperforming companies, pressuring them to reform and selling out for a fat profit. In recent years, his targets have included Apple Inc., eBay and Dell Inc. His efforts have made him one of America's richest people: Forbes magazine puts his net worth at more than $20 billion, making him the 18th-wealthiest American.
In the 1980s, he pioneered so-called greenmail raids in which financiers threatened companies with hostile takeovers unless they were paid a premium to go away.
Walters is a legendary figure in sports betting circles, widely feared by sports book operators as one of the few people who can consistently win. He's bet millions on Super Bowls alone, and told "60 Minutes" in a 2011 profile that he has never had a losing year. An early user of computer data, Walters was one of the few bettors whose opinion was so respected that he could move point spreads if it was known what side he was betting on.
Walters and a group of bettors dubbed The Computer Group were indicted in the mid-1980s for running what prosecutors said was a bookmaking operation, but were acquitted at trial. Walters was also indicted on money laundering charges in 1998 and had $2.8 million in cash confiscated from a safe deposit box, but the charges were later dismissed and the money returned.
Walters was also a high stakes gambler on the golf course, regularly playing celebrities or PGA Tour pros for cash. He told Golf Digest that he once lost a $2 million bet and once made a 40-foot putt worth $400,000. Walters teamed up with touring pro partner Fredrik Jacobson to win the AT&T Pebble Beach National Pro-Am by 10 strokes in 2008 while playing as an 11 handicap.
Walters also owns golf courses and auto dealerships. Politically connected in Las Vegas, Walters is also known for his philanthropy, particularly toward Opportunity Village, which trains developmentally disabled adults.
Mickelson, 43, was inducted into the World Golf Hall of Fame in 2011. He goes to the U.S. Open next month in North Carolina with a chance to become only the sixth golfer to capture all four major championships.
Mickelson has long had a reputation to gamble, though he has said he scaled back his habit after his son, Evan, was born in 2003 following a troublesome pregnancy. The most publicized payoff was when Mickelson and friends won $560,000 on a preseason bet (28-1 odds) that the Baltimore Ravens would win the 2001 Super Bowl.
On the golf course, he has a long history of playing money games during the practice rounds. He occasionally gets a group of players and caddies together for dinner and small wagering during the NBA and NHL playoffs, and prominent fights.
A year ago, Mickelson was criticized for public comments that tax increases in California kept him from being part of the San Diego Padres' new ownership group and might cause him to leave his native state. He said his federal and state taxes amount to over 60 percent.
Associated Press writers Paul Wiseman and Eric Tucker in Washington and AP Sports Writer Tim Dahlberg in Las Vegas contributed to this report.