FBR Analyst: Banks may have to raise more capital

As the number of bad loans mount, bank regulators will likely continue to pressure many companies to raise capital, Miller said.

"We again caution investors who believe that higher non-performing assets are already factored into financial stock valuations and have become somewhat complacent about future credit losses," he wrote in a note to clients. "We remain very cautious on financials in general, and we would continue to sell most institutions with rapidly growing NPAs relative to equity."

Miller ranked banks and thrifts with a market capitalization above $500 million by non-performing assets as a percentage of tangible equity and allowances for loan losses. A year ago, just three had non-performing assets above 12 percent of capital and allowance, but now the top 20 all exceed that threshold, with five over 20 percent, he said.

Ratios will only get worse, given falling home prices and rising defaults," he wrote. "Given continued growth in NPAs relative to capital, all of the companies at the top of our ranking are at risk of having to raise additional capital."

Washington Mutual Inc. shares fell 36 cents or 6.5 percent to $5.18 in afternoon trading. SunTrust Banks Inc. rose 20 cents to $37.23; Regions Financial Corp. fell 26 cents, or 2.3 percent, to $11.32; and Webster Financial Corp. fell 40 cents, or 2 percent, to $19.37.