NEW YORK – Goldman Sachs stepped up its defense against civil fraud charges Monday, telling clients it did not withhold information in a complex transaction involving risky mortgage securities.

But a big question was: Will other big investment banks face similar charges?

In a letter to clients, Goldman Sachs Group Inc. vowed to fight the government’s charges that the bank and one of its vice presidents misled investors by selling complex financial products tied to mortgages that were expected to fail. Both Goldman Sachs and the vice president, Fabrice Tourre, were named in the Securities and Exchange Commission lawsuit Friday.

The SEC charged that Goldman Sachs did not tell two clients that the investments they bought were crafted by billionaire hedge fund manager John Paulson, who was betting on them to fail.

“Goldman Sachs would never condone one of its employees misleading anyone, certainly not investors, counterparties or clients,” the bank said in the letter to clients.

The bank fought back against the government’s charges, saying, “The SEC does not contend that the two professional institutional investors involved did not know what they were buying, or that the securities included in this privately placed transaction were in anyway improper.”

The two investors, the German bank IKB Deutsche Industriebank AG and the financial consulting firm ACA Management LLC, “were provided extensive information about those securities and knew the associated risks,” Goldman Sachs said.

The bank also said it lost $90 million on the deal, but it did not explain how it incurred the loss.

Meanwhile, many observers, believing that other banks were involved in similar deals, wondered whether more companies would be charged.

An SEC spokesman declined to say Monday whether firms other than Goldman Sachs are under investigation. But financial experts say it’s likely that other big names could find themselves being questioned.

“Wall Street is full of copycats,” said John Coffee, a securities law professor at Columbia Law School. “Once you know which deals you’re talking about, it’s not hard to see who else was doing it.”

At the height of the housing boom, several big banks packaged and sold investments tied to mortgage securities to meet investor demand. Such securities, called synthetic collateralized debt obligations, or CDOs, reaped huge fees for banks but later were blamed for creating the credit crisis and worsening the recession.

Among the biggest sellers of CDOs were Merrill Lynch & Co., now part of Bank of America Corp.; Citigroup Inc.; Swiss banking giant UBS; and German bank Deutsche Bank, according to a banking industry official.

Citigroup denied any involvement in the transaction Goldman Sachs is charged in connection with. But Citigroup said it has talked with the SEC during the agency’s probe of the role of derivatives in the financial crisis. “We are fully cooperating with these investigations, and it would not be appropriate for us to comment further,” said John Gerspach, Citigroup’s chief financial officer.

A Citigroup spokesman declined to comment on whether the SEC has indicated that it is pursuing civil charges against the bank. That information would be part of what’s known as a Wells notice, a letter describing probable charges against a company and requesting a response.

According to news reports, Goldman Sachs received such a notice in June 2009 and did not disclose the notice to investors. Such a move is required only if the bank believes the notice would have a material impact on its business or shareholders.

Bank of America has not received a Wells notice regarding Merrill Lynch, a person with knowledge of the matter said. UBS and Deutsche Bank declined to comment.

The SEC charges don’t contest the securities’ legality; rather, the charges challenge how they were sold. Goldman Sachs told the buyers that ACA Management had selected the pools of subprime mortgages it used to create the securities.

But the SEC alleges that Goldman Sachs misled the buyers by failing to disclose that Paulson’s hedge fund, Paulson & Co., also played a role in selecting the mortgage pools and stood to profit from their fall in value.

IKB and a second bank, ABN Amro NV, lost a total of about $1 billion on the investments. Another loser was the Royal Bank of Scotland, which acquired major portions of ABN Amro in 2007. The SEC said the Royal Bank of Scotland paid Goldman $841 million to unwind ABN transactions.

IKB lost nearly all of its $150 million investment, the SEC said. Most of the money the banks lost went to Paulson in a series of transactions between Goldman and the hedge fund, the agency said.

IKB, an early casualty of the financial crisis, was sold in 2008 to Dallas-based Lone Star Funds.

In its letter to clients, Goldman Sachs said ACA Management had “rejected numerous securities suggested by Paulson & Co., including more than half of its initial suggestions, and was paid a fee for its role as portfolio selection agent in analyzing and approving” the portfolio of CDOs.

For Goldman Sachs, which is scheduled to report first-quarter earnings today, the stakes in the case couldn’t be higher. The worst-case scenario would be if it were to lose its license to do business. That could actually happen if the company lost a jury trial and any subsequent appeals. More realistically, experts said, the company is likely to settle the case.

“Goldman’s management is not going to put the future or fate of Goldman on the roulette wheel,” said Jacob Frenkel, a former SEC enforcement lawyer now with the Shulman Rogers law firm in Potomac, Md. “But that does not mean it will not fight, at least for some period.”