Citigroup is investigating a rumor that one of its traders entered a trade that helped precipitate a drop of almost 1,000 points in the Dow Jones Industrial Average, a spokesman for the bank said on Thursday.

Citigroup, the third-largest U.S. bank, currently has no evidence that an erroneous trade has been made, the spokesman said.

Earlier, sources told Reuters that the plunge in the Dow Jones Industrial average -- its biggest intraday point drop ever -- may have been caused by an erroneous trade entered by a person at a big Wall Street bank.

Market sources said the erroneous trade may have involved shares of the so-called E-Mini, a stock market index futures contract that trades on the Chicago Mercantile Exchange's Globex trading platform. The composition of the E-Mini is similar to the stocks in the S&P 500.

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

Other market sources said the erroneous trading involved the IWD exchange-traded fund or the S&P 500 Mini. 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 (1847 GMT), 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.

(Additional reporting by Dan Wilchins, Deepa Seetharaman and Maria Aspan; Editing by Gary Hill)

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