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Shortly after 2:15 p.m. Eastern time on Thursday, hedge fund Universa Investments LP placed a big bet in the Chicago options trading pits that stocks would continue their sharp declines.

On any other day, this $7.5 million trade for 50,000 options contracts might have briefly hurt stock prices, though not caused much of a ripple. But coming on a day when all varieties of financial markets were deeply unsettled, the trade may have played a key role in the stock-market collapse just 20 minutes later.

The trade by Universa, a hedge fund advised by Nassim Taleb, author of "Black Swan: The Impact of the Highly Improbable," led traders on the other side of the transaction—including Barclays Capital, the brokerage arm of British bank Barclays PLC—to do their own selling to offset some of the risk, according to traders in Chicago.