Mary Schapiro, chairman of the Securities and Exchange Commission, said major Wall Street firms contributed to the stock market plunge last week by retreating from the market when support for normal trading was needed most. Schapiro discounted many of the theories suggested for the cause of the panic, including the idea of a "fat finger" trade, terrorist activity, hacking and derivatives trading. "The markets failed investors," Schapiro said. But "at this point, we are unable to point to a single event which could be the sole cause." Schapiro also said the SEC has issued subpoenas to investigate the event.