Mickelson in insider trading probe
The winner of five major championships said he has done "absolutely nothing wrong".
A federal official briefed on the investigation said he FBI and Securities and Exchange Commission are analysing trades Mickelson and Las Vegas gambler Billy Walters made involving Clorox at the same time activist investor Carl Icahn was attempting to take over the company.
Smiling as he stood before a room packed with reporters and cameras, Mickelson said the investigation had not been a distraction until FBI agents approached him after his opening round on Thursday. He did not offer any other details, including his relationship with Walters or any stock advice he might have received.
He said it would not affect his preparations for the US Open in two weeks, the only major he lacks for the career Grand Slam.
"It's not going to change the way I carry myself," Mickelson said after an even-par 72 at Muirfield Village left him far behind the leaders. "Honestly, I've done nothing wrong. I'm not going to walk around any other way."
The federal official told the AP that Mickelson and Walters placed their trades at about the same time in 2011. Federal investigators are looking into whether Icahn shared information of his takeover attempt of Clorox with Walters, and whether Walters passed that information to Mickelson.
The New York offices of the US Attorney and the FBI declined to comment.
Newspaper reports said federal officials also were examining trades by Mickelson and Walters involving Dean Foods in 2012.
After a brief interview, Mickelson stepped outside and signed autographs for about 20 minutes, like it was any other day at a tournament. Fans were supportive as ever on the golf course, and Mickelson gave away so many golf balls to children that his caddie had to ask a tour official to retrieve more balls from his locker when they reached the turn.
He had released a statement earlier, and said that he would not release further details.
"I can't really go into much right now, but as I said in my statement, I have done absolutely nothing wrong," Mickelson said. "And that's why I've been fully cooperating with the FBI agents, and I'm happy to do in the future, too, until this gets resolved."
He declined to answer questions about his relationship with Walters, who owns several golf courses. And when asked whether Walters advised him to invest in Clorox or Dean Foods, Mickelson matter-of-factly replied to a Wall Street Journal reporter: "You should know. You wrote the article."
Icahn, 78, is one of Wall Street's most successful corporate raiders, famous for buying stock in under-performing companies, pressuring them to reform and selling out for a fat profit.
In recent years, his targets have included Apple, eBay and Dell. His efforts have made him one of America's richest people: Forbes magazine puts his net worth at more than 20 billion dollars (£12bn), 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 high-stakes gambler and developer who owns The Walters Group. The company operates several golf courses. He has said he earns 15 million dollars (£9m) a year gambling on sports.
Mickelson, 43, was inducted into the World Golf Hall of Fame in 2011. He goes to the US 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.
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 betting during the NBA and NHL play-offs, and prominent fights.
A year ago, Mickelson was criticised 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%.