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5 things we still don't know about the market plunge

It's also impossible to find out when exactly the crash started. Was it 2:40 p.m.? Or 2:45? Maybe 2:30? 2:37? Considering there are hundreds of thousands of trades each minute, that seemingly small distinction is important.

2. Why has no one come forward to take responsibility?

The problem with the "fat-finger trade" theory is this: No firm or person has claimed that the trade was theirs. In fact, several firms, including Citigroup and Terra Nova Financial, issued categorical denials. Refusing to take responsibility is highly unusual behavior for trading firms: Especially in a market crash, it looks particularly craven. To some, the blankness of the faces involved is a good argument for more regulation of "dark pools" that trade securities far away from the prying eyes of the exchanges. But this is also the primary reason that some suspect market manipulation or a cyberattack.

3. Why did the exchanges cancel trades if they insist there was no glitch?

The New York Stock Exchange and Nasdaq denied that there was any technological glitch in their trading, meaning that the bizarre trades were due to forces outside their control. The exchanges could be telling the absolute truth, or they could be avoiding some embarrassing assumptions people might make in light of past, similar technological problems. (Both exchanges have histories of pricing glitches and blackouts.) Nonetheless, both exchanges unilaterally decided that they would cancel all trades in stock whose prices changed more than 60 percent -- the largest single cancellation of stock trades in history.

To traders, this makes no sense: If there were a provable glitch, the cancellations would be fine. But if there was no technical glitch and the system as a whole was just doing its job, then the exchanges interfered with bargain shopping.

4. Why can't the exchanges get their stories straight?

The New York Stock Exchange and Nasdaq are losing market share to electronic exchanges and upstart competitors. They also compete with each other and have a long history of smack-talking on the court.

It's probably no wonder, then, that the first reaction of each was to get into a hand-flapping tizzy blaming the other. Nasdaq blamed the NYSE for walking away from stock trading. The NYSE blamed Nasdaq and others for not jumping in when its own systems slowed down trading. The NYSE, resentful about the loss of many of its floor traders in the rush to technology, also used the excuse as an occasion to settle some scores, strangely insisting that the problem with all these computers is exactly why the NYSE needs to have floor traders.

Perhaps true, but saying "we need humans because our computers don't work" is not a solid defense. Besides, people didn't do great a job of reining things in, either.

5. What is the connection between the stocks that crashed?

There are almost no common characteristics among the biggest stock movers that day: Procter & Gamble is a consumer stock, Accenture is a financial stock; Boston Beer Company sells Sam Adams beer. The e-minis theory partially explains the connections because it's possible for unrelated stocks to appear in a single trading "basket."

In addition, it's unclear why exchange-traded funds, or ETFs, made up the majority, or 69 percent, of the canceled trades. That's too high a number to be a coincidence, but why these funds? It might be because most ETFs are created to track particular indexes, such as the Dow or S&P 500, which experienced wild swings. But as usual on this subject, answers -- not guesses -- are in short supply.

Bonus: Can this happen again?

We're assuming it can. But it would be nice to know for sure.

If Tuesday's congressional hearing could have clarified even one or two of these issues, it would have been a success. Instead, yet another hearing is being scheduled.

Heidi N. Moore, a financial journalist in New York City, is a former reporter for the Wall Street Journal.

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