In a statement, the head of the Securities and Exchange Commission, Mary L. Schapiro, said that the major exchanges, including the New York Stock Exchange and Nasdaq had “agreed on a structural framework, to be refined over the next day,”
The plunge was an unintended consequence of a system built to place a circuit breaker on stocks in sharp decline. In theory, trades slow down so that sellers can find buyers the old-fashioned way, by hand, one by one. The electronic exchanges did not slow down in tandem, causing problems, according to two officials familiar with the investigation.
That could mean that the computers first flooded the market with sell orders that could not be matched with buyers. Then, just as quickly, many of these networks withdrew from trading. The combined effect might have set off a chain reaction that sent shares of many companies spiraling during the 15-minute frenzy.
After a weekend of analysis, many specialists at the major exchanges no longer believe that a single large sell trade in one stock, like that of Procter & Gamble, was the trigger, according to the people familiar with the investigation. Instead, they suspect that a mismatch in rules between the older New York Stock Exchange and younger electronic exchanges set off a frightening sequence of events.