WASHINGTON — Now that the federal government has thrown a lifeline to mortgage giants Fannie Mae and Freddie Mac, taxpayers could be on the hook for billions more if the crisis of confidence spreads.

There were encouraging signs yesterday for the rescue plan, but also signs of concern — notably on Wall Street, where shares of the two companies slumped further — that the plan won't be enough.

Other banks are already teetering: National City Corp. shares fell nearly 15 percent on rumors of financial trouble, even though it said it was experiencing no unusual depositor or creditor activity. And Washington Mutual Inc.'s shares fell 35 percent, to a paltry $3.23 amid worries about whether it had enough cash to handle the mortgage market downturn. WaMu said it did.

And worried customers lined up yesterday to pull cash out of their accounts at IndyMac Bank, seized on Friday by the federal government.

Some critics said they fear the Fannie-Freddie rescue effort will make more bailouts inevitable by sending a message that some institutions are too big to fail and thus encouraging risky behavior.

"It sends the wrong message to the world," said Joshua Rosner, managing director of research firm Graham, Fisher & Co. in New York.

Sung Won Sohn, an economics professor at The Smith School of Business at Cal State Channel Islands, cited soaring oil costs, a weakening economy and an unstable housing market that he said will only get worse.

"I don't think these steps are enough to arrest the deterioration," he said.

As long as more homeowners default on mortgages, losses to financial institutions will mount. Those losses already exceed $400 billion, and some analysts believe they will top $1 trillion before the housing carnage is over.

By comparison, Congress has authorized $650 billion so far to fight the Iraq war.

The Bush administration and the Federal Reserve announced an emergency rescue plan Sunday to bolster Fannie Mae and Freddie Mac, which hold or guarantee more than $5 trillion in mortgages — almost half of the nation's total.

The plan would temporarily increase a long-standing Treasury line of credit that could be provided to either company. Treasury also said it would, if necessary, buy stock in the companies to make sure they have enough money to operate.

The Fed also announced it would allow Fannie and Freddie to get loans directly from the Fed — a privilege previously granted only to commercial banks until this March, when the Fed extended the borrowing to investment banks to deal with the collapse of Bear Stearns.

Meanwhile, hundreds of worried customers lined up yesterday to pull their money out of IndyMac bank, seized by the government Friday in the second-biggest bank failure in U.S. history.

The Federal Deposit Insurance Corp. estimated the IndyMac failure, the largest since the collapse of Continental Illinois in 1984, would cost between $4 billion and $8 billion out of the agency's $53 billion insurance fund.

Analysts do not expect the volume of bank failures that happened from 1990 to 1992, when 834 of them folded. But the FDIC does plan to review whether to raise the fees it charges banks to beef up its insurance fund.

Brian Bethune, chief U.S. financial economist at Global Insight, called the troubles at Fannie and Freddie a "potentially dangerous turn of events" for the U.S. economy.

If Fannie and Freddie were unable to play their huge roles in financing new mortgages, the housing market would only suffer more, said Mark Zandi, chief economist at Moody's www.Economy.com — and would create turmoil for financial institutions around the world that invest in Fannie and Freddie's debt securities.

Fannie and Freddie's financial reports remain difficult to understand, even after accounting scandals that came to light five years ago forced the companies to restate several years of earnings and oust top executives.

The next few weeks — during which Fannie and Freddie post their second-quarter results and may attempt to raise a bigger capital cushion — are key, Zandi said. He said the best possible outcome is if the rescue plan helps the two companies stabilize their finances on their own without any loss of government loans.

"At the end of the day, with a little bit of luck, it won't cost taxpayers a dime," Zandi said.