Did "fat finger" record a gigantic, but bogus, trade, setting off Thursday's selloff?

Today's near-1,000 point selloff in the Dow Jones Industrial Average ($INDU) was reportedly caused by a "fat finger" mistake by a trader at Citigroup (C).

CNBC reported that numerous sources said that a trader entered a "b" instead of an "m" for million in a trade possibly involving Procter & Gamble (PG), a Dow component. The trader reportedly placed a sell order of $16 billion, instead of $16 million, worth of e-minis, futures contracts tied to equity indexes.

Citigroup said that it has "no evidence of an erroneous trade" but that it is investigating.

P&G, which closed at $62.12 per share on Wednesday, tanked to $39.37 this afternoon, before rebounding to close down $1.44, or 2.3%, at $60.72.

P&G said that it was looking into the sudden drop in the price of its stock. P&G also said that it is working with the U.S. Securities and Exchange Commission about what caused the plunge in its stock price.

Each stock has its own circuit-breaker level, according to the New York Stock Exchange. When the stocks fall below that level, they can then be traded on any other exchange or platform at any price.

So when P&G fell below its circuit breaker, a bid came in for the stock at $39.37 from the Nasdaq, the NYSE said, according to MarketWatch. However, P&G did not go below $56 on the NYSE.

In periods of exacerbated volatility, the NYSE goes into "slow mode," NYSE Euronext CEO Duncan Niederauer explained to CNBC. Instead of trading in miliseconds, the NYSE takes 30 seconds or a minute to seek out liquidity. When that happens, the other exchanges do not have to honor NYSE quotes, they don't have to wait for the stock to reopen, he said. P&G was closed for 90 seconds on the NYSE.

"I don't think we're looking for some bank who mistakenly sold $2 billion when they meant to sell $2 million. I actually don't think that had anything to do with it," Niederaurer said.

I think we're going to find that actually very little volume traded in those 60 to 90 to 120 seconds."