The pressure in the less-liquid markets was amplified by the computer-driven trades, which led still other traders to pull back. Only when traders began to manually respond to the sharp drop did the market seem to turn around, said the official, who spoke on the condition of anonymity because the investigation was not complete.

On Friday evening, another government official directly involved in the investigation said that regulators had not yet been able to completely rule out any of the widely discussed possible causes of the market’s gyrations.

Image Traders applauding Duncan L. Niederauer, chief of NYSE Euronext, on Friday. He had defended his exchange in an interview. Credit... Richard Drew/Associated Press

This official, who also spoke on the condition of anonymity, said that regulators had collected statistical and trading data from stock and futures exchanges, and had begun cross-analyzing that with trading reports from brokerage firms and large market participants. Regulators have also gathered anecdotal accounts of what happened from hedge funds and other trading firms.

The two major regulatory agencies — the Securities and Exchange Commission and the Commodity Futures Trading Commission — have generated multiple memos detailing what they have found and offering possible causes for the market events. Among the issues discussed in the memos, the official said, were the disparate rules that different stock exchanges have for dealing with large price movements on the same securities and how prices on futures markets and stock exchanges appeared to lead or follow each other’s movements down and back up.

The lack of a firm answer, more than 24 hours after the market’s plunge Thursday, left some on Wall Street frustrated.

“The problem is you don’t come in and find out what the clear answer is,” said Art Hogan, the New York-based chief market analyst at Jefferies & Company. “We don’t have the clear explanation for how it happened.”