Accelerating the declines, high-frequency hedge funds, which use computers to trade at super high speed, appeared to pull back from the market as prices collapsed.

WSJ: Dow Takes a Harrowing 1,010.14-Point Trip

Others worried about the hyperactivity of high-frequency traders have suggested charging them for each trade they break, although that would not necessarily improve liquidity and is an idea that is unlikely to be pursued.

ECONOMIST: The post mortem into the stockmarket slide of May 6th

Regulators are looking into whether exchanges, in a rush to gain the business of high-frequency firms, have provided advantages to some sophisticated trading firms that allow them to trade profitably at the expense of other investors, according to people familiar with the continuing probes.

WSJ: High-Speed Trading in the Spotlight

He says many ETF firms court high-frequency traders, which help provide the buy and sell orders that can make ETFs easier to trade for other investors.

WSJ: High-Speed Trading in the Spotlight