NEW YORK — The upheaval in the American financial system sent shock waves through the stock market Monday, producing the worst day on Wall Street in seven years as investors digested the failure of one of its most venerable banks and wondered which domino would be next to fall.
The Dow Jones industrial average lost more than 500 points, more than 4 percent, its steepest point drop since the day the stock market reopened after the Sept. 11, 2001, attacks. Retirement plans, government pension funds and other investment portfolios lost $700 billion.
The wreckage capped a tumultuous 24 hours that redrew U.S. finance. Lehman Brothers, an investment bank that predates the Civil War and weathered the Great Depression, filed the largest bankruptcy in American history. A second storied bank, Merrill Lynch, fled into the arms of Bank of America.
It was by far the most stomach-churning single day since a financial crisis began to bubble up from billions of dollars in rotten mortgage loans that have crippled the balance sheets of one bank after another and landed mortgage giants Fannie Mae and Freddie Mac under the control of the federal government.
"We are in the middle of a deep, dark recession, and it won't end soon. Here it is, and it is pretty nasty," said Barry Ritholtz, who writes the popular financial blog The Big Picture and is CEO of research firm FusionIQ.
And the fallout was far from over. American Insurance Group, the world's largest insurer, was fighting for its very survival: New York Gov. David Paterson moved to allow the company to tap one of its subsidiaries for an emergency loan to stay above water.
"AIG still remains financially sound," Paterson said, even as the company's stock tumbled along with many others.
The Dow industrials dropped 504.48 points to close at 10,917.51, the first time since July they have finished under 11,000. It was the sixth-largest point drop ever and the worst since Sept. 17, 2001, when the average fell 684.81 points on the first day of trading after the terror attacks.
The index has shed nearly a quarter of its value since its record high last October.
Financial stocks fell as investors worried about the strength of banks' balance sheets. Washington Mutual Inc. fell 27 percent to $2 a share, while Wachovia Corp. fell 25 percent to $10.71.
While Lehman Brothers was filing for Chapter 11 and AIG was scurrying to find financing to stay afloat, Merrill Lynch was avoiding a similar fate with a $50 billion transaction to become part of Bank of America Corp.
The deal would create a financial giant rivaling Citigroup Inc., the biggest U.S. bank in terms of assets. Bank of America has the most deposits of any U.S. bank, while Merrill Lynch is the world's largest and most widely recognized brokerage.
"It was an opportunity of a lifetime," said Ken Lewis, Bank of America's chairman and CEO.
One huge concern is that the Lehman bankruptcy will probably trigger even tighter credit — making it more difficult for everyone from large companies to small businesses to American homebuyers to borrow money.
In marathon sessions Friday night, Saturday and Sunday, government officials and the chief executives of major U.S. and foreign banks huddled at the New York Fed's fortress-like building in downtown Manhattan, trying to work out a way to save Lehman.
They failed at that. But a group of 10 banks that includes JPMorgan, Goldman Sachs and Citigroup formed a $70 billion pool that banks or brokerages can tap to cover short-term funding needs.
There were also worries that Lehman's problems would infect other financial companies and spread to global stock markets, further harming the U.S. and global economies.
The Fed meets Tuesday to decide interest rate policy. It's widely expected to keep rates at 2 percent, but some economists believe it could lower them to soothe Wall Street's frazzled nerves.