A large investor using an automated trading software to sell futures contracts sparked the brief-but-historic stock market “flash crash” on May 6, according to a report by federal regulators released Friday. In the 104-page report, staff members at the Securities and Exchange Commission and the Commodity Futures Trading Commission said an unnamed investor used a trading algorithm to sell orders for futures contracts called E-Minis, which traders use to bet on the future performance of stocks in the S&P 500 index. The contracts were sold quickly and in large numbers, according to the report, on a day when the market was already under stress due to concerns about the European debt crisis.