According to the Journal, the investigation also involves Carl Icahn, the 78-year-old billionaire who runs the publicly traded Icahn Enterprises, and William "Billy" Walters, a famed Las Vegas bettor and real-estate mogul.
None of the three has been accused of any wrongdoing, and no charges have been filed.
"I have done absolutely nothing wrong," Mickelson said in a statement early Saturday morning, hours after playing in the second round of the Memorial Tournament. "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 newspaper, citing people briefed on the probe, reported that the Federal Bureau of Investigation and the Securities and Exchange Commission are looking into whether Mickelson and Walters traded nonpublic information they allegedly received from Icahn about his investments in public companies. The FBI and SEC are reviewing the trading patterns of both Mickelson and Walters as part of an investigation that began three years ago when Icahn made a $10.2 billion offer for Clorox that caused the stock to jump, the report said.
According to the Journal, two FBI agents approached Mickelson after his opening round at the Memorial tournament Thursday in Dublin, Ohio, to discuss the investigation. Mickelson told them to speak to his attorney. FBI agents also spoke to Mickelson last year at Teterboro Airport in New Jersey about his trading, according to The New York Times.
Glenn Cohen, a lawyer for Mickelson, told the Journal on Friday: "Phil is not the target of any investigation. Period." Cohen also said an FBI agent has told him that Mickelson is not a target of the investigation.
Icahn also denied knowledge of the probe, telling the Journal, "We do not know of any investigation. We are always very careful to observe all legal requirements in all of our activities."
According to the Journal, Icahn and Walters have a past friendship. Mickelson and Walters, meanwhile, play golf together, and on occasion Walters has suggested stocks for Mickelson to consider buying, the newspaper reported, citing a person familiar with their relationship. Icahn says he doesn't know Mickelson.
Mickelson shot 72 and 70 in the opening two rounds of the Memorial and is tied for 37th. He tees off for the third round at 10:27 a.m. ET on Saturday.
Mickelson, 43, is second to Tiger Woods in career earnings on the PGA Tour, with $73.7 million in official prize money in his 22-year career. But he is also a prolific earner off the course; Golf Digest reported that Mickelson had $45 million in endorsement income in 2013.