As he hit a few practice balls before his round at the Memorial Tournament in Dublin, Ohio, Mickelson was in a light-hearted exchange with fellow American golfer Robert Garrigus.
"How's it going, Phil?" asked Garrigus.
"It's been an interesting evening," Mickelson replied, adding with a laugh, "I don't have much to say about it."
Garrigus jokingly intimated that talking with Mickelson could get him into trouble, saying he was "not sure I want to talk to you now."
On Friday night, a source familiar with the matter said the FBI and the U.S. Securities and Exchange Commission were investigating possible insider trading involving Icahn, Mickelson and Walters.
Federal investigators are looking into whether Mickelson and Walters may have traded illegally on private information provided by Icahn about his investments in public corporations, the source told Reuters, confirming reports on Friday.
None of the three men has been accused of any wrongdoing, the source said.
Mickelson, 43, a three-times Masters champion, winner of 42 tournaments on the PGA Tour and one of the world's highest-paid athletes, said in a statement issued through his manager early on Saturday: "I have done absolutely nothing wrong. I have cooperated with the government in this investigation and will continue to do so."
He declined to elaborate with reporters after his round at the tournament on Saturday, saying: "Hopefully shortly we will be able to discuss this further, but for right now I can't."
The Wall Street Journal cited Glenn Cohen, Mickelson's lawyer, as saying the golf star was not a target of the federal probe.
There was plenty of support among fans at the tournament, with members of the crowd shouting: "Go Phil!" and "We're here for you, Phil."
Some fans, however, were dismayed at news of the probe.
"Phil is the everyman's man, even though he's mega-rich," said David Hughes, of Los Angeles. "He shares his wealth with great causes, thrills thousands of fans almost every week and with all the wolves of Wall Street, they want to go after him? Give me a break."
Icahn, an activist investor, told Reuters he was unaware of any investigation and said his firm always followed the law. He acknowledged a business relationship with Walters, but said he did not know Mickelson personally.
"I am very proud of my 50-year unblemished record and have never given out insider information," Icahn said.
Walters and Mickelson play golf together, the source familiar with the investigation told Reuters.
Walters did not respond to requests for comment. Officials with the FBI and the SEC declined to comment.
Mickelson, a native of California who took up the game as a toddler, has piled up career earnings of more than $73 million and considerably more via corporate endorsements and his golf course design company.
According to Forbes, he is seventh on its list of the world's highest-paid athletes, second only to Tiger Woods among golfers. In the 12 months to July 2013, his total earnings were $48.7 million, Forbes said.
His sponsors include Barclays PLC, Exxon Mobil Corp , KPMG and Amgen. Barclays and Exxon Mobil declined to comment on the insider trading probe and many of his other top sponsors could not be reached for comment.
The player clinched his fifth major title at last year's British Open. If he wins next month's U.S. Open, it would complete a career grand slam of the four majors.
Asked if he could put the probe past him leading into the U.S. Open, Mickelson said: "Yes I do. you know as a player you have to be able to block out whatever is going on off the golf course and focus on the golf course and it's not going to change the way I carry myself.
"Honestly I have done nothing wrong. I am not going to walk around any other way."
The insider trading investigation began three years ago, according to the source. It is the latest to emerge from a multi-year insider trading crackdown by U.S. authorities.
The investigation centers on suspicious trades in Clorox Co by Walters and Mickelson as Icahn was trying for access to the board of the consumer products company in 2011, the New York Times reported, citing people briefed on the probe.
Icahn had accumulated a 9.1 percent stake in Clorox in February 2011. In July, he made an offer for the company that valued it at above $10 billion and sent its stock soaring.
Investigators were also looking into trades that Mickelson and Walters made related to Dean Foods Co, the Journal cited the people as saying. The New York Times cited people briefed on the investigation as saying that in that particular case, investigators were looking into trades placed around 2012, just before the company announced quarterly results.
Those trades appeared to have no connection to Icahn, the newspaper added. Icahn told Reuters he had never purchased shares or been involved with Dean.
Federal prosecutors in Manhattan are handling the inquiry in conjunction with the FBI and the SEC, the New York Times reported. Since August 2009, Manhattan U.S. Attorney Preet Bharara's office has convicted 81 people of insider trading, either at trial or via guilty pleas, with no acquittals.
The current investigation made little headway initially, the Times reported. Investigators are searching phone records, seeking to determine whether Icahn had spoken to Walters before the trades, the Times cited anonymous people briefed on the probe as saying.
About a year ago, FBI officials approached Mickelson at Teterboro Airport in New Jersey, asking him to discuss his trading, the Times cited its sources as saying.
Icahn, a prolific tweeter and vocal critic of some of America's largest corporations, habitually broadcasts his thoughts on corporations and, occasionally, stock positions he has taken.
Even if Icahn did leak information about his plans regarding Clorox, it may not necessarily have violated the law. Insider trading regulations prohibit trading based on material, nonpublic information obtained from someone who breached a fiduciary or confidentiality duty by disclosing it.
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