By Jay Heflin - 05/07/10 03:37 PM EDT
Regulators originally thought a trading glitch caused the market to freefall, but Greifeld believes that it was only one aspect. The fear that financial unrest in Greece could be spreading to other European countries, and heightened activity in the futures market that spilled into the equity market also contributed to the issue.
During yesterday's chaos, several stocks quickly lost nearly all of their value. Accenture PLC was reduced to a penny, from $40 a share, and Proctor & Gamble stock lost 35 percent of its value in 2 minutes.
Greifeld said many of the trades that occurred during the crisis would be reversed.
"For those trades that are cancelled, it is as if they did not happen," he said.