Washington ? The Federal Reserve and Treasury Department struggled Monday to contain the fallout from an upheaval among the country’s largest investment banks as they moved on to their next challenge – engineering a $75 billion private rescue of the nation’s largest insurance company.

The insurer, American International Group, faces a cash crunch that grew more severe Monday night when the major credit-rating agencies warned investors that the company could have greater difficulty in meeting its obligations. It was unclear whether the downgrades by the agencies would force AIG to post additional collateral at a time when it is having difficulty raising money.

Investors sent the Dow Jones industrial average plunging more than 500 points, or 4.4 percent, for the biggest point loss since the Sept. 11 terrorist attacks seven years ago. About $700 billion in shareholder value disappeared in a single day of trading.

The wrenching reshaping of Wall Street – which over the weekend included the demise of one big firm and the sale of another – also pushed the value of the dollar lower. It sent the price of crude oil below $100 a barrel for the first time since Feb. 15 as traders bet a global downturn would reduce the demand for energy.

Wall Street’s biggest shakeout since the Great Depression stems from a collapse in housing prices, which spread losses among firms that bet on securities linked to mortgages. Twice in the past year, regulators intervened to save financial firms and prevent further erosion in the housing markets. But over the weekend, officials drew the line at rescuing the storied investment bank Lehman Brothers, which Monday filed for bankruptcy protection.

“We had a very, very tough day on the market,” said Art Hogan, chief market analyst at Jefferies & Co. “Investors are anxious about the spillover effect of Lehman and what is the next shoe to drop.”

As investors digested the news, some economists worried whether Wall Street’s troubles were spilling over into other parts of the economy, renewing pressure on the Federal Reserve to cut interest rates when it meets today.

Fed leaders, however, believe it is too early to tell what the impact might be, and they are unlikely to cut rates for now.

In the meantime, Treasury Secretary Henry M. Paulson Jr. signaled Monday that taxpayer funds could still be used broadly to “maintain the stability and orderliness of our financial system” but that he was pressing healthier Wall Street firms and commercial banks to join together to assist in rescuing individual firms – much like the purchase of Merrill Lynch on Sunday by Bank of America.

Goldman Sachs, for instance, was asked by the Federal Reserve Bank of New York to help AIG, a $1 trillion-asset insurance company that serves 74 million consumers in 130 countries. AIG had been heavily involved in the business of issuing complex insurance contracts to investors in securities backed by mortgages, and the collapse of subprime and other home loans threatened to hobble the company and trigger a chain reaction in the financial system.