Once bitten, twice shy.

As banks rack up billions of dollars in losses from bad loans and blundered investments, large investors are becoming skittish about pumping more money into them.

In the past several weeks, bank executives have encountered unexpected resistance from investors, who have expressed reluctance to participate in the capital-raising transactions sweeping through the industry, according to people familiar with the situation. Already bruised by big losses and fearing that bank shares haven't yet hit bottom, some of these investors are choosing to tighten their purse strings.

"The window for capital-raising is closing," says Brad Evans, a portfolio manager for Heartland Advisors Inc., a money-management firm in Milwaukee that invests in small, regional banks. "Investing in a bank right now means investing in a large portfolio of loans that are essentially a black box."

The change in sentiment could have sweeping implications for financial institutions that are trying to shore up their balance sheets by issuing stock and other securities to their investors. Some may be forced to lure investors with sweeter terms, further raising the costs of doing these deals.