SAC Capital Advisors LP’s clients pulled $1.68 billion from hedge fund as Steven A. Cohen’s firm is being investigated for alleged insider trading, according to a person familiar with the matter.

The hedge fund had more than $15 billion in assets under management at the start of the year, said the person, asking not to be named because the information is private. SAC told some employees and advisors last month it expected client redemptions of at least $1 billion.

Clients account for about 40 percent of SAC’s assets under management and the rest is from Cohen and his employees. SAC this week reached a deal with Blackstone Group LP that gives all clients three more months to decide whether to stay in the fund.

The hedge fund was told by the U.S. Securities and Exchange Commission in November that the agency is considering pursuing civil fraud claims against it, related to alleged insider trading in two drugmakers by former portfolio manager Mathew Martoma. Blackstone, one of the biggest investors in SAC, with about $550 million in the fund, will leave most of the money in place for at least another quarter under the new liquidity agreement, it said yesterday in a statement.

The new terms give SAC investors time to see if the fund and its billionaire founder are charged as part of a multiyear government probe that has already linked at least eight current or former employees to allegations of insider trading at the firm.

New Terms

Clients faced a deadline of yesterday to tell SAC, which is based in Stamford, Connecticut, if they wanted to start the process of withdrawing all their money by the end of the year. Under existing rules, they could redeem 25 percent of their assets from the firm each quarter.

Now, SAC is telling investors they can wait until mid-May to make the decision. At that time, they can redeem a third of their money each in the second, third and fourth quarters.

“While we submitted redemptions for certain accounts as appropriate, BAAM successfully preserved flexibility for our clients by extending our decision time line,” Peter Rose, a spokesman for New York-based Blackstone, said in the statement. BAAM is the firm’s hedge-fund unit, Blackstone Alternative Asset Management.

Blackstone negotiated the new terms with the hedge fund over the past week, said people familiar with SAC, who asked not to be named because the firm is private. SAC obtained board approval within two days of the redemption deadline, they said.