A long streak of speculative lending got out of hand as banks and even staid industrial companies made a stream of risky loans. Consumer spending on cars and clothes was slipping, but no one was paying attention. The stock market grew shaky in September, and then in October, the bottom fell out.

Suddenly, everyone seemed to want to sell. But there were few buyers, and over six bleak trading days, the Dow Jones Industrial Average lost a third of its value. It was a panic, said a senior New York Stock Exchange official, "where...