NASDAQ, the SEC, and others are attempting to piece together what caused a three-hour freeze in trading late last week, reports The Wall Street Journal. Sources say that officials are focusing particularly on two minutes about an hour before the shutdown, a period that could have signaled that major trading problems were coming. Between 10:53am and 10:55am, the NYSE Arca exchange apparently tried to connect to NASDAQ 30 times and failed, while NASDAQ was also having difficulty connecting to Arca. Though the problems seemed to have been fixed soon after, the multiple failed attempts are said to have overloaded NASDAQ's computers, stopping them from updating stock quotes and eventually making trading impossible.

NASDAQ issued a statement after the freeze noting problems connecting to another system, though traders took issue with its CEO's handling of the situation. After the original problem was fixed, NASDAQ had to delay reopening for hours as it tried to avoid crashing its system again with a sudden glut of trades. And problems continued even after the market was reopened, though not at a catastrophic level. The fragile system of high-frequency trading has often proved vulnerable to small glitches, which can snowball into larger crises like Facebook's IPO debacle — which NASDAQ eventually paid $10 million to settle. Accordingly, investigators look for events that could portend a later problem, allowing them to stop trading and avert it.