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 U.S. Federal Bureau of Investigation and the Securities and Exchange Commission are 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 have been accused of any wrongdoing, the source said.
Mickelson, a 42-times winner 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."
"I wish I could fully discuss this matter, but under the current circumstances it's just not possible," the statement said.
The Wall Street Journal cited Glenn Cohen, Mickelson's lawyer, as saying the golfing legend was not a target of the federal probe.
Mickelson, 43, was warmly received by the crowd at the first tee on Saturday and was business as usual - hitting a booming drive down the middle of the fairway.
There was plenty of support, 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, a legendary 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. Spokespeople for 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 BARC.L, Exxon Mobil Corp XOM.N, KPMG and Amgen AMGN.O. Barclays declined to comment on the insider trading probe, and there was no comment yet from any of the other sponsors.
The player clinched his fifth major title at last year's British Open. If he won next month's U.S. Open, he would complete a career grand slam of the four majors.
YEARS OF INVESTIGATING
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 CLX.N 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 DF.N, 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 are 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 nor 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.
And 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.
(Additional reporting by Jennifer Ablan and Mark Lamport-Stokes; Writing by Edwin Chan and Frances Kerry; Editing by Peter Henderson and Gunna Dickson)
By Jennifer Ablan