The Securities and Exchange Commission said Tuesday that it would temporarily institute circuit breakers on all the stocks in the Standard & Poor's 500-stock index after the huge market gyrations on May 6. The circuit breakers will pause trading in those stocks for five minutes if the price moves by 10 percent or more in a five-minute period. The trial run will begin after a 10-day comment period and will last through Dec. 10, the commission said. The circuit breakers will apply both to rising and falling stock prices. But in a separate report, the SEC and the Commodity Futures Trading Commission said that they had not been able to pinpoint the cause of the sharp market decline that shook investors and markets two weeks ago. Generally, the agencies said, the drop was caused by traders stepping back from the market and refusing to buy or sell, in both the stock and futures markets. The agencies said they had found no evidence that the market decline was caused by so-called fat finger trading errors, or by computer hacking or terrorist activity. They added, however, 'we cannot completely rule out these possibilities.' The SEC said it worked with the national securities exchanges and the Financial Industry Regulatory Authority to shape the new rules. The rules require approval by the five-member commission after the 10-day comment period. After the trial, the exchanges will re-evaluate the trading curbs and perhaps expand them to all stocks permanently. But the exchanges also will continue to use different rules about slowing trading in some shares. The New York Stock Exchange said it had independently elected to continue its system of 'liquidity replenishment points,' which slow trading in stocks that move a certain amount in a certain time period. Those measures are used across the entire Big Board's listings, while the proposed individual circuit breakers apply only to stocks in the S.&P. 500. The SEC also said it intended to further review the role of the so-called self-help mechanism, under which one exchange can refuse to route orders to another if it perceives that orders are not being filled quickly enough. Stock and stock-index futures prices were already declining on May 6 when, about 2:42 p.m. Eastern time, they suddenly plunged by more than 5 percent over the next five minutes. The review found that almost 200 stocks briefly lost almost all their value during the flash crash, trading in many cases for a single penny. The investigation identified 11,510 trades in which shares were sold for less than 10 percent of their previous value, involving more than 3.5 million shares. The shares were worth $212.4 million at 2:40 p.m. Over the next 20 minutes, they sold for $557,516. 