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The embattled hedge fund SAC Capital Advisors is bracing itself for another round of withdrawal requests from investors after disclosing that it would no longer fully cooperate with the government’s scrutiny of its trading practices.

SAC’s investors have two weeks to decide whether to withdraw money from the $15 billion hedge fund, which is owned by Steven A. Cohen. Earlier this year, SAC investors asked to redeem $1.7 billion, and the firm is scrambling to stanch the outflow of more funds as fears rise that the insider trading investigations could further damage Mr. Cohen and his firm.

In the latest blow, SAC’s largest outside investor, the Blackstone Group, is preparing a request to withdraw a portion of its money before the June 3 deadline, according to people briefed on the matter. Blackstone could take out as much as half of its roughly $550 million investment, these people said.

As one of the world’s largest hedge fund investors, Blackstone is viewed as an industry bellwether, and others could look to it for guidance on whether to keep their money at SAC.

That said, Blackstone has not decided to terminate its relationship with the firm. And on Monday at least two other investors — although both significantly smaller than Blackstone — reiterated their support for Mr. Cohen, who has one of the best investment track records on Wall Street.

“I’m very comfortable and confident having my money with him,” said Ed Butowsky, managing partner of Chapwood Investments in Dallas, a firm that invests client money in SAC. “All I know is that the returns are coming in nice, and my clients are happy.”

Happy does not describe the mood inside SAC. Mr. Cohen, 56, and his colleagues are said to be highly agitated by the government’s latest move in its multiyear investigation, according to people with direct knowledge of the firm. Last week, prosecutors sent SAC a new set of subpoenas, requesting a variety of documents and issuing a subpoena to Mr. Cohen to testify before a grand jury.

Hedge Fund Inquiry

SAC employees are viewing the government’s latest subpoenas as oppressive and reflective of a prosecutor’s office — led by Preet Bharara, the United States attorney in Manhattan — that is determined to shut down the firm. The firm has already agreed to pay $616 million to resolve two civil insider trading lawsuits. It also issued a broad set of reforms to improve its compliance practices. However, at least nine former SAC employees have been tied to insider trading while at the fund; four have pleaded guilty.

SAC, which until last week fully cooperated with the lengthy investigation, resisted some of the government’s latest requests. And the firm disclosed to its investors late last week that it was no longer cooperating unconditionally with the inquiry.

Mr. Cohen, who has not been charged with any wrongdoing, is expected to exercise his constitutional right against self-incrimination under the Fifth Amendment. He sat earlier this year for government questioning in a civil insider trading case being investigated by the Securities and Exchange Commission, but his lawyers objected to his sitting for open-ended, unlimited questioning in a criminal grand jury setting.

It is unclear whether prosecutors in the United States attorney’s office in Manhattan will force Mr. Cohen to appear before a grand jury and assert his Fifth Amendment right. Typically, when a witness’s lawyer has indicated that the client refuses to testify, the office’s practice is to not require an in-person appearance. But in certain cases, prosecutors will demand that the person come in for the interview.

A legal deadline looms for prosecutors to bring a criminal case against Mr. Cohen related to charges against Mathew Martoma, a former SAC portfolio manager accused of illegally trading in the shares of two drug companies, Elan and Wyeth. The Martoma case is the first time that Mr. Cohen was linked to questionable trades, which occurred in late July 2008. Under the five-year statute of limitations for insider trading crimes, the government must charge Mr. Cohen by July.

Yet the eliciting of Mr. Cohen’s grand jury testimony is not entirely bad news for the hedge fund manager, at least as it relates to his criminal exposure, legal experts say. A grand jury subpoena seeking Mr. Cohen’s testimony suggests that the government is pursuing a case against SAC, but not Mr. Cohen himself. It is highly unusual for prosecutors to issue a grand jury subpoena to the target of an investigation, indicating that they want to interview Mr. Cohen broadly about his fund’s activities.

But bringing criminal charges against SAC would also be an unusual move by the government. Over the last decade, the Justice Department has moved away from indicting companies after the 2002 indictment of Arthur Andersen was widely seen as having put the accounting giant out of business.

Ellen Davis, a spokeswoman for the United States attorney’s office in Manhattan, declined to comment.

The flurry of new subpoenas comes at an inconvenient time for SAC. Investors in the fund have two weeks to decide whether to ask for their money back, or else they will have to wait until mid-August to make a withdrawal request.

SAC has several built-in protections to prevent a mass exodus of money that could cripple the firm. For one thing, roughly 60 percent — or $9 billion — of the $15 billion under management belongs to Mr. Cohen. And there are stringent limits on client withdrawals; if they ask for their money back, they will get it back in three installments over the next three quarters.

The fund’s executives have aggressively tried to prevent withdrawals since November, when the Securities and Exchange Commission warned the fund that it faced possible civil actions related to insider trading. After agreeing to pay $616 million this year to settle two civil insider-trading actions brought by the agency, SAC’s lawyers said they believed that much of the firm’s legal problems were in the rearview mirror.

Mr. Cohen has doggedly maintained that he wants to continue to run his fund and manage other people’s money. Yet as the harsh legal spotlight on him intensifies and prominent investors like Blackstone reduce their commitments, Mr. Cohen could wind down SAC and manage his own billions in a so-called family office structure. Other hedge fund tycoons have followed this path, like George Soros and his protégé, Stanley Druckenmiller.

A group of Mr. Cohen’s investors continue to stand by him and hope that he stays in business. For Anthony Scaramucci, chief executive of the hedge fund firm SkyBridge Capital and a friend of Mr. Cohen’s, sticking with SAC has as much to do with friendship and loyalty as it does its superior performance.

“A lot of guys, when bombs are going off, you figure out very quickly who your friends are in the trenches,” Mr. Scaramucci said. “Most friends run from bullets, but your best friends run toward them. I have enormous amount of respect for the guy, and I think he’s misunderstood.”