Washington ? A trading firm’s use of a computer sell order triggered the May 6 market plunge, which sent the Dow Jones industrial average careening nearly 1,000 points in less than a half-hour, federal regulators said Friday.

A report by the Securities and Exchange Commission and the Commodity Futures Trading Commission determined that the so-called “flash crash” occurred when the trading firm executed a computerized selling program in an already stressed market.

The firm’s trade, worth $4.1 billion, led to a chain of events that ended with market players swiftly pulling their money from the stock market, the report said.

The report does not name the trading firm. But only one trade that day fit the description in the report. The firm Waddell & Reed, based in Overland Park, Kan., has acknowledged making such a trade that day.

The free-fall highlighted the complexity and perils of the fast-evolving securities markets.

Electronic trading platforms now compete with the traditional exchanges. Stocks are traded on about 50 exchanges beyond the New York Stock Exchange and the Nasdaq Stock Market. Computers using mathematical formulas give so-called “high frequency” traders a split-second edge. Electronic errors at high speeds can ripple through markets.

The stock market was already stressed even before the plunge that day. Anxiety was mounting over the debt crisis in Europe.

The Dow Jones had been down about 2.5 percent at 2:30 p.m., when the trader placed an enormous sell order on a futures index of the S&P’s index, called the E-Mini S&P 500. The trade was automated by a computer algorithm that was trying to hedge its risk from price declines.

In that one trade, 75,000 contracts were sold within 20 minutes. It was the largest trade of that investment since the start of the year. The firm’s previous transaction of that size took more than five hours, the report notes. The trade triggered aggressive selling of the futures contracts and that sent the index sinking about 3 percent in four minutes.

The report said the design of the firm’s trading formula may have amplified the rush to sell.

It said the formula ignored price changes and responded to the volume of trading. The automated program sped up the firm’s selling as other market players began trading the first block of futures contracts that flooded the market.

Other notable business news:

Stocks nudge higher as manufacturing improves. Stocks started off October on a positive note following mostly good news on the economy. Shares of big manufacturing companies like Boeing Co., General Electric Co. and 3M Co. rose Friday after the Institute for Supply Management said its manufacturing index showed that factory activity was still expanding in September, although not quite as fast as analysts had hoped and slightly slower than the month before.

• U.S. auto sales remain sluggish despite new models. New models and Labor Day promotions didn’t do much to fire Americans’ appetites for new cars in September. Sales at Chrysler Group LLC and Ford Motor Co. rose slightly from August. They fell at General Motors Co. and Honda Motor Co. and were flat at Toyota Motor Corp.