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.
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And about a year ago, FBI officials approached Mickelson at Teterboro Airport in New Jersey, asking the celebrity golfer 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. The billionaire took to Twitter earlier this year when he pushed Apple Inc's board to expand the iPhone maker's buyback program, and several times tweeted when he had increased his position in the tech company.
During a campaign to get eBay Inc to hive off its fast-growing PayPal division, he openly criticized the Internet retail giant's board. But the activist investor has tweeted just once in the past three weeks.
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.
As a non-board member, Icahn owed no duty to Clorox shareholders. It is possible, though by no means definite, that Icahn owed a confidentiality duty to his own shareholders.