Citigroup’s total E-mini volume for the entire day was only 9 billion, suggesting that the origin of the trades was elsewhere, according to someone close to Citigroup’s own probe of the situation. The E-minis trade on the CME.

A CME spokesman said it found no problems with its systems.

The Nasdaq and New York Stock Exchange took the unusual step of declaring that they would cancel some trades that took place during the height of the selloff. Both markets said they will cancel all trades more than 60 percent above or below market that occurred between 2:40 p.m. and 3:00 p.m. New York time.

Other market sources said the erroneous trading involved the IWD exchange-traded fund or the S&P 500 Mini, according to Reuters. A person close to BlackRock, which manages the IWD, said there was no unusual trading in the iShares product.

Amid the sell-off, Procter & Gamble shares plummeted nearly 37 percent to $39.37 at 2:47 p.m. EDT, prompting the company to investigate whether any erroneous trades had occurred.

The shares are listed on the New York Stock Exchange, but the significantly lower share price was recorded on a different electronic trading venue.

"We don't know what caused it," said Procter & Gamble spokeswoman Jennifer Chelune. "We know that that was an electronic trade...and we're looking into it with Nasdaq and the other major electronic exchanges."

A different P&G spokesman had said earlier the company contacted the Securities and Exchange Commission, but Chelune said that he spoke in error.

One NYSE employee leaving the Big Board's headquarters in lower Manhattan said the P&G share plunge lay at the center of whatever happened.

"I'll give you a tip," the employee said, speaking on condition of anonymity. "P&G. Check out the low sale of the day. Something screwed up with the system. It traded down $30 at one point."

Nasdaq said it was working with other major markets to review the market activity that occurred between 2:00 p.m. and 3:00 p.m., when the market plunge happened.

The exchange later said it was investigating potentially erroneous transactions involving multiple securities executed between 2:40 and 3:00 p.m.

Nasdaq also said participants should review their trading activity for potentially erroneous trades.

The massive selloff, which began shortly after 2 p.m. ET, amplified concerns about the spreading European debt crisis as the approval of austerity measures by the Greek Parliament sparked renewed rioting in Athens.

"There is simply a growing recognition that Greece has got to default," banking analyst Dick Bove told CNBC.com. "The riots in the streets showed the decision to repay the debt was not going to be made by the people in Germany, France and Switzerland—it's going to be made by people in Greece and they're not going to repay it."

There also is a growing sense that any collapse of Greece could trigger a wave of defaults across Europe and even the world.

"We've seen a crisis start in a country—Greece—become regional, impact the whole of the Euro zone and is on the verge of truly going global," El-Erian, CEO of the world's biggest bond fund, told CNBC shortly before the selloff began. (Click here for story)

—Reuters contributed to this report