They're still stymied about what caused traders to panic this week, sending Wall Street into a 1,000-point descent, U. S. regulators have said.

The New York Times reported on Friday that officials from the Securities and Exchange Commission and other regulatory panels initially focused on how high-speed trading networks interact among themselves and with exchanges such as the New York Stock Exchange.

Regulators shifted away from a theory that it was a trading mistake and were looking into links between the futures and cash markets for stocks, an official involved in the investigation said.

They were looking at how different trading rules governing different exchanges could have contributed to the problem, the SEC and the Commodity Futures Trading Commission said in a joint statement issued after trading on Thursday.

The statement said, "We are scrutinizing the extent to which disparate trading conventions and rules across various markets may have contributed to the spike in volatility."

Regulators hadn't eliminated any theory for the market's dive, another government official said on Friday.

The official told the Times regulators have begun cross-analyzing data from stock and futures exchanges with trading reports from brokerage firms and large market participants. Officials also recorded anecdotal accounts of Thursday's events from hedge funds and other trading firms.

Art Hogan, chief market analyst at Jefferies & Co., told the Times, "The problem is you don't come in and find out what the clear answer is. We don't have the clear explanation for how it happened." (With Inputs from Agencies)