The 2009 wiretap on Mr. Cohen's home, where the noted art collector keeps a yellow Jeff Koons balloon dog sculpture on his lawn, produced little in the way of incriminating evidence and was discontinued, said the people briefed on the investigation, who were not authorized to discuss it. These same people said that previous reports that authorities had wiretapped Mr. Cohen's home phone in 2008 were erroneous.
Another area of frustration for investigators has been a lack of instant message conversations involving Mr. Cohen. The hedge fund owner, like most others at SAC, was a frequent user of AOL instant messaging services during 2007 and 2008, the period during which the undercover investigation began gathering steam. During the trading day, the best way to get Mr. Cohen's attention was to send him an instant message, the people briefed on the matter said. But the hedge fund, as a general rule, did not save I.M. conversations before 2009.
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In the 2009 phone call, which was not recorded in a wiretap, Mr. Cohen told Mr. Grodin that he believed Richard Choo-Beng Lee, a technology stock trader, might be cooperating with federal authorities, even taping telephone calls with people he knew in the industry, said people briefed on the matter who did not want to be identified. Mr. Cohen suggested that Mr. Lee, who had once worked for both men, might be trying to implicate others, the people briefed on the matter said.
Mr. Grodin, who had worked for SAC Capital five years earlier, was surprised. He doubted that Mr. Lee, known as C. B. to his friends, would cooperate with federal authorities in an investigation of the industry, the people briefed on the matter said.
But by October 2009, Mr. Cohen's warning about Mr. Lee would ring true when federal prosecutors disclosed that he had been working with them for months, taping conversations with people in connection with an investigation that led to the arrest of the hedge fund billionaire Raj Rajaratnam that month.
Mr. Cohen, 57, has not been accused of any criminal wrongdoing. A spokesman for Mr. Cohen declined to comment.
A lawyer for Mr. Grodin did not return calls seeking comment.
(Read more: Jury finds SAC Capital Advisors' Michael Steinberg guilty)
Federal authorities continue to pursue Mr. Cohen, who was widely regarded on Wall Street as one of the best stock traders of his generation. Last month, his firm pleaded guilty to insider trading and agreed to pay a $1.2 billion penalty and stop managing money for wealthy investors, pensions and institutional investors.
The closest authorities have come to tying Mr. Cohen to improper trading at his firm is a civil failure-to-supervise action filed against Mr. Cohen by the Securities and Exchange Commission.
But the phone call to Mr. Grodin suggests that Mr. Cohen may have had more insight in 2009 into the investigation being conducted by the United States attorney's office in Manhattan and the F.B.I. than others in the hedge fund business did. The investigation has led to the convictions or guilty pleas of 77 traders, analysts and industry consultants — including seven people who worked for Mr. Cohen's fund.
It is unclear exactly when Mr. Cohen phoned Mr. Grodin, but the call appears to have coincided with an attempt by Mr. Lee to persuade Mr. Cohen to give him a job at SAC, where he had worked from 1999 to 2004. Mr. Lee was seeking either a full-time job or a consulting job with SAC, shortly after he began secretly cooperating with federal investigators in April 2009. Federal authorities had Mr. Lee call and tape the conversation with Mr. Cohen.
A person briefed on the matter who was not authorized to speak publicly about the investigation said Mr. Lee's call to Mr. Cohen was useful in enabling federal authorities to persuade a federal judge to approve a brief wiretap on Mr. Cohen's mansion.