Federal prosecutors are conducting a criminal investigation into whether Goldman Sachs or its employees committed securities fraud in connection with its mortgage trading, people familiar with the probe say.

The investigation from the Manhattan US Attorney's Office, which is at a preliminary stage, stemmed from a referral from the Securities and Exchange Commission, these people say. The SEC recently filed civil securities-fraud charges against the big Wall Street firm and a trader in its mortgage group. Goldman and the trader say they have done nothing wrong and are fighting the civil charges.