Investigators seeking an explanation for the brief stock market panic last week said Sunday that they were focusing increasingly on how a controlled slowdown in trading on the New York Stock Exchange, 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.

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