Diamondback Capital Management, for instance, a $3 billion hedge fund, told its investors that 14.9 percent of its assets were locked up in the Lehman bankruptcy — money it could not extract. A number of other hedge funds were in the same predicament. (When called for comment, Diamondback officials did not respond.)

As this news spread, every other hedge fund manager had to worry about whether the balances they had at other Wall Street firms might suffer a similar fate. And Morgan Stanley and Goldman Sachs were the two biggest firms left that served this back-office role. That is why Mr. Ackman’s investors were calling him. And that is what caused hedge funds to pull money out of Morgan Stanley and Goldman Sachs, hedge their exposure by buying credit-default swaps that would cover losses if either firm couldn’t pay money they owed — or do both.

It was fear, not greed, that was driving everyone’s actions.

Breaking the Buck

There was another piece of bad news spooking investors — and government officials. On Tuesday, the Reserve Primary Fund, a $64 billion money market fund, and two smaller, related funds, revealed that they had “broken the buck” and would pay investors no more than 97 cents on the dollar.

Money market funds serve a critical role in greasing the wheels of commerce. They use investors’ money to make short-term loans, known as commercial paper, to big corporations like General Motors, I.B.M. and Microsoft. Commercial paper is attractive to money market funds because it pays them a higher interest rate than, say, United States Treasury bills, but is still considered relatively safe.

A run on money funds could force fund managers to shy away from commercial paper, fearing the loans were no longer safe. One reason given by the Reserve Primary Fund for breaking the buck was that it had bought Lehman commercial paper with a face value of $785 million that was now worth little because of its bankruptcy. If money market funds became fearful of buying commercial paper, that would make it far more difficult for companies to raise the cash needed to pay employees, for instance. At that point, it would not just be the credit markets that were frozen, but commerce itself.

Just as important, in the eyes of federal officials, was that money market funds had long been viewed by investors as akin to bank accounts — a safe place to store cash and earn interest on that money. Despite lacking federal deposit insurance, these funds held $3.4 trillion in assets.

“Breaking the buck was the Rubicon,” said a federal official. “This was the first time in the crisis that you could see stories talking about how it was affecting real people.”