The accusations amount to a black eye for the once-untouchable Goldman Sachs, a money machine that is the epicenter of Wall Street power. For decades, its platinum reputation has attracted top investors and stock underwriting deals.

Several of its former chief executives have gone on to high public office, among them Henry M. Paulson Jr., the former Treasury secretary, and Jon Corzine, the former New Jersey governor. (Henry Paulson and John Paulson are not related.)

In recent months, Goldman has been defiant in the face of criticism, repeatedly defending its actions in the mortgage market, including its own bets against it. In a letter published last week in Goldman’s annual report, the bank rebutted criticism that it had created, and sold to its clients, mortgage-linked securities that it had little confidence in.

“We certainly did not know the future of the residential housing market in the first half of 2007 any more than we can predict the future of markets today,” Goldman wrote. “We also did not know whether the value of the instruments we sold would increase or decrease.”

The letter continued: “Although Goldman Sachs held various positions in residential mortgage-related products in 2007, our short positions were not a ‘bet against our clients.’ ” Instead, the trades were used to hedge other trading positions, the bank said.

Goldman was one of many Wall Street firms that created complex mortgage securities — known as synthetic collateralized debt obligations — as the housing wave was cresting. At the time, traders like Mr. Paulson, as well as those within Goldman, were looking for ways to bet against the overheated market.

For months, S.E.C. officials have been examining mortgage bundles like Abacus that were created across Wall Street. The commission has been interviewing people who structured Goldman mortgage deals about Abacus and similar instruments. The commission advised Goldman that it was likely to face a civil suit in the matter, sending the bank what is known as a Wells notice several months ago.