U.S. regulators probing the May flash crash are focusing on a trading practice known as "quote stuffing", in which large numbers of rapid-fire orders to buy or sell stocks are placed and canceled almost immediately.

CFTC commissioner Scott O'Malia told Reuters on Thursday that the futures regulator was reviewing data from Nanex LLC, a trade database developer that issued a study suggesting that computer algorithms used quote stuffing to gain an edge during the May 6 crash.

The U.S. Securities and Exchange Commission, which is investigating the crash jointly with the Commodity Futures Trading Commission, is looking at quote stuffing and something called "sub-penny pricing", a person familiar with the flash crash probe said.

The Nanex study uses market graphics and playful names to illustrate quote stuffing, arguing that high-frequency trading firms do this to flood the marketplace with bogus orders to distract rival trading firms.

Investors could make trades under the false impression that those orders were legitimate, only to see liquidity disappear and the market move against them when the orders are canceled -- all in the blink of an eye.