Market Inquiry Focuses on One Trader

She cautioned against drawing too direct a conclusion, however. “It must be recognized, however, that the fact that stocks prices follow futures prices chronologically does not demonstrate what may have triggered the price movements,” she said.

Mr. Gensler also pointed to several factors in the market for individual stocks that could have led to the market’s decline, including the overall turbulence in the global economy, particularly in Europe, which had made traders edgy.

The two agencies said Tuesday that they would form a joint committee of academics, market participants and former regulators to address regulatory questions raised by last Thursday’s market turmoil.

Ms. Schapiro said the S.E.C. had issued several subpoenas to unidentified people or entities to help gather information related to the market crash. She said regulators needed the subpoenas to gather information from nonregulated entities, a group that would include some hedge funds.

Ms. Schapiro said the enormous increase in trading volume in recent years was complicating efforts to draw clear conclusions about the plunge.

When a special commission examined the market crash of 1987, for example, investigators looked at a day when trading on the New York Stock Exchange totaled about 600 million shares. Last Thursday, 10.3 billion shares of stocks listed on the exchange changed hands.

Ms. Schapiro said the S.E.C. had already begun to examine whether high-frequency traders, whose computer-driven systems trade millions of shares of stocks daily, should be subjected to stricter trading rules to protect market integrity and quality.

The evolution of electronic stock markets in recent years has resulted in high-frequency traders taking over many of the market functions previously filled by stock exchange specialists and market makers who provided the bulk of stock market liquidity, Ms. Schapiro said.

The major stock exchanges also testified to the subcommittee on Tuesday, with each generally pointing fingers at the others.

In its testimony, the Nasdaq exchange pointed to technical problems related to the New York Stock Exchange’s electronic trading system called Arca.

During the market chaos this caused three exchanges to stop routing orders to the New York Stock Exchange because of what they saw as slowness in response times or unreliable quotes being posted by the exchange.

The New York Stock Exchange says there was no problem with its technology. But market experts said the decision by the exchange’s rivals — Nasdaq, BATS Trading and CBOE — to make that move added to the nervousness of high-frequency traders who abruptly withdrew from the market.

“The normal stabilizers were pushed out by the systems glitches,” said James J. Angel, a professor at Georgetown University who studies financial markets.