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May 6, 2010, started out as an ordinary day on Wall Street. Then around a quarter to 3, the Dow Jones Industrial Average plunged nearly 600 points within the space of just a few minutes before correcting itself. What caused the so-called flash crash has been debated ever since. This week, U.S. officials blame the episode on a little-known trader who worked from his parents' home in London using computer software that he'd modified himself. NPR's Jim Zarroli reports.
JIM ZARROLI, BYLINE: The flash crash seemed to embody everyone's worst nightmare about the mysteries and risks of high-speed computer trading. Michael Greenberger is a former regulator at the Commodity Futures Trading Commission.
MICHAEL GREENBERGER: This was a worldwide threat to economic well-being, and it only lasted a matter of minutes.
ZARROLI: The arrest this week of 36-year-old Navinder Singh Sarao has only underscored how opaque the market remains. Sarao was no Wall Street titan. He was a lone trader working out of the West London neighborhood of Hounslow. Milto Savvidis, a trader who worked with him, described him this way.
MILTO SAVVIDIS: He's a legend in our firm, we'd say. And during the financial crisis, this guy, for the lack of a better word, had balls. He'd just used to get into big positions, and he saw the risk, he saw the reward, and he took on the trades.
ZARROLI: U.S. officials say over the years Sarao used a technique known as spoofing to manipulate trading in the futures market. They say he was able to drive the price of futures contracts up or down by setting up a huge number of trades he never completed. Again, Michael Greenberger.
GREENBERGER: I mean, that would look like Goldman Sachs was shorting the market. There were so many trades, but he didn't have to worry about putting any money up because he had orchestrated the software so when it ever came to the point that he actually had to buy the contract that would sell the market short, he disappeared.
ZARROLI: Officials say once the price was where he wanted it, Sarao would trade, reaping big profits. He allegedly made $40 million over four years. Greenberger says Sarao was well-known to regulators.
GREENBERGER: And the interesting thing is he was always in trouble with the Chicago Mercantile Exchange. In fact, the very day of the crash, he received a letter from the exchange complaining about his activities.
ZARROLI: But it took U.S. officials five years to publicly tie Sarao to the crash, and they only did so after getting tipped off by a whistleblower. Eric Hunsader of Nanex has spent years studying the flash crash.
ERIC HUNSADER: I'm just shocked that the CFTC, when they were analyzing this data, never saw this. We saw it within - I think it was one of the first things we noticed when we started analyzing it back in December of 2010.
ZARROLI: Instead, a congressional report attributed the crash to computer trading by a large mutual fund company later identified as Waddell & Reed. But that claim was always controversial and questions remain about who was really to blame. Hunsader says it's unlikely that Sarao was responsible for the crash all by himself. For one thing, on the afternoon of May 6, he shut down his software program two-and-a-half minutes before the crash began - an eternity in high-speed trading.
HUNSADER: I'll just use an analogy of a forest fire. He was the one who brought the kindling, the wood, if you will. We still need the gasoline and we still need the match.
ZARROLI: U.S. officials want to extradite Sarao, which could take a long time. Even if he can block the request, his troubles aren't over. Sarao is also known to have traded on a German exchange, and European regulators are also said to be looking into his activities. Jim Zarroli, NPR News, New York.
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