WASHINGTON — The government yesterday accused Wall Street's most powerful firm of fraud, saying Goldman Sachs & Co. sold mortgage investments without telling the buyers that the securities were crafted with input from a client who was betting on them to fail.

And fail they did. The securities cost investors close to $1 billion while helping Goldman client Paulson & Co., a hedge fund, capitalize on the housing bust. The Goldman executive accused of shepherding the deal allegedly boasted about the "exotic trades" he created "without necessarily understanding all of the implications of those monstrosities!!!"

The civil charges filed by the Securities and Exchange Commission are the government's most significant legal action related to the mortgage meltdown that ignited the financial crisis and helped plunge the country into recession.

The news sent Goldman Sachs shares and the stock market reeling as the SEC said other financial deals related to the meltdown continue to be investigated. It was a blow to the reputation of a financial giant that had emerged relatively unscathed from the economic crisis.

Goldman Sachs denied the allegations. In a statement, it called the SEC's charges "completely unfounded in law and fact" and said it will contest them.

The SEC is seeking to recoup the money lost by investors and impose unspecified civil fines against Goldman Sachs and the executive, Fabrice Tourre.

The charges come as lawmakers seek to crack down on Wall Street practices that helped cause the financial crisis.