Wall Street took a late dive after the United States government said it was starting criminal and civil investigations into the Gulf of Mexico oil spill.

Regulators plan to enact new rules to curb the type of market volatility behind the May 6 "flash crash" that caused the Dow to plunge almost 1,000 points in a half hour. The goal is to head off market gyrations that can be exacerbated by automated high-speed trading.

The Dow Jones Industrial Average plummeted almost 1,000 points in a half-hour today, the cause of which appears to be human error exacerbated by a market made more volatile by high-speed trades and automatic sale orders that are measured in milliseconds.

The stock market crash on Thursday afternoon took financial Web sites down with it, as people hurried online to make trades and check their investments.

National Australia Bank Ltd has called a trading halt pending an "imminent" announcement about the status of negotiations to buy the Australian and New Zealand businesses of AXA Asia Pacific.