It was a stock market mystery that had everyone guessing for months: just what caused that harrowing flash crash last May?

On Friday, after months of investigation and speculation, federal authorities finally provided the answer: it all began with the click of a computer mouse in Kansas.

In a long-awaited report on one of wildest days in Wall Street’s history, regulators said that the automated sale of a large block of futures by a mutual fund  not named in the report, but identified by officials as Waddell & Reed Financial, of Overland Park, Kan.  touched off a chain reaction of events on May 6. The Dow Jones industrial average plunged more than 600 points in a matter of minutes that day and then recovered in a blink.

The finger-pointing and speculation that followed  Were high-speed traders behind it? A rogue computer program? Financial terrorists?  captivated Wall Street. But in the report released on Friday, the authorities said they found no evidence of market manipulation. Instead, the temporary crash resulted from a confluence of forces after a single fund company tried to hedge its stock market investment position legitimately, albeit in an aggressive and abrupt manner.