With so much uncertainty, some investors are abandoning banking shares, fearing shareholders will be wiped out if the government seizes control. The worry, investors say, is that Washington is running out of time and options.

“Banks live on confidence, and there is precious little coming from the new Treasury secretary,” said Gary B. Townsend, a former federal banking regulator who runs his own investment firm, referring to Timothy F. Geithner. “We are getting only confusion.”

Bank of America and Citigroup, both of which have received tens of billions of dollars in government aid, bore the brunt of the selling on Friday. Bank of America sank 14 cents, to $3.79, while Citigroup fell 56 cents, to $1.95, although both rallied in after-hours trading. The decline capped the stock market’s worst week since November, when Citigroup confronted a crisis of confidence that eventually compelled it to reach for a second financial lifeline from Washington. The Dow Jones industrial average fell 100.28 points, to 7,365.67 Friday.

Bankers were hoping the government would clarify its intentions. But several said that senior administration officials with whom they had been in regular contact had not spoken to them in days. Others fear a further, rapid market decline might force the government to act, although banking regulators insist the daily gyrations of the stock market will not force their hand. Instead, officials are focusing on whether bank customers are withdrawing money or lenders are struggling to finance themselves with short-term borrowing. So far, there have been no such warning signs.

One piece of the administration’s plan — a new “stress test” for the nation’s banks — could become clearer next week. Regulators are preparing to apply the test to assess the health of 20 or so large banks, and are expected to provide more details about the process on Wednesday.