The financial crisis entered a potentially dangerous new phase on Wednesday when many credit markets stopped working normally as investors around the world frantically moved their money into the safest investments, like Treasury bills.

As a result, the cost of borrowing soared for many companies, while the stocks of Wall Street firms like Goldman Sachs and Morgan Stanley that only a couple of weeks ago were considered relatively strong came under assault by waves of selling. Investors were so worried that they snapped up three-month Treasury bills with virtually no yield and they pushed gold to its biggest one-day gain in nearly 10 years. Stocks fell by nearly 5 percent in New York.

The stunning flight to safety, away from other kinds of debt as well as stocks, could cause serious damage to an already weakened economy by making it more expensive for businesses to finance their daily operations.

Some economists worry that a psychology of fear has gripped investors, not only in the United States but also in Europe and Asia. While investors’ decision to protect themselves may be perfectly rational, the crowd behavior could cause a downward spiral with broader ramifications.