Investigators seeking an explanation for the brief stock market panic last week said on Sunday that they were focusing increasingly on how a controlled slowdown in trading on the New York Stock Exchange (NYSE), meant to bring about stability, instead set off uncontrolled selling on electronic exchanges.
It 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.
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 NYSE and younger electronic exchanges set off a frightening sequence of events.
It is not known exactly what caused the initial sell-off in the blue chips, but investigators say the earliest sign of trouble they have found was a sudden drop in the value of a futures contract on the Chicago Mercantile Exchange, based on the Standard & Poor's 500-stock index.