"The public knows that there are always risks involved in investing in the stock market," Spitzer said in a statement. "But part of the risk should not include questions about whether trades will be executed promptly or whether online brokerage firms can deliver on the services that they've promised." Investors funnelled a record 340,000 trades a day through the Internet in the fourth quarter, up 38 percent from the third quarter, and trading volumes in January are rising at the same clip, according to industry reports.

About one in seven stock trades now takes place online. Internet brokers have signed up some 7 million customers, and many expect that number to rise to more than 10 million at this year's end.

The surge in Internet trading, however, has led to many system outages at brokers, frustrating many investors.

Major Internet brokerages Charles Schwab Corp., E+Trade Group Inc. and AmeriTrade Holding Corp. all have reported outages and software glitches recently. Many E+Trade customers were unable to trade on Wednesday because of an outage and one investor reported problems again Thursday morning.

The glitches already have attracted the attention of regulators, prompting U.S. Securities and Exchange Commission chairman Arthur Levitt to warn cyberspace investors that they can end up paying more for a stock than they thought because of system delays and Internet shares' wild price swings.

Stocks such as those of Internet media company Yahoo Inc. can gain or lose tens of dollars in the space of an hour.

Spitzer's office said it had fired off letters to several online firms, asking them to provide information about their services. A spokesman was not immediately available to name the firms subject to the inquiry.

"Based on the tremendous growth of online trading, and a corresponding increase in complaint calls to my office, this is an issue we must look into," said Spitzer. "Unfortunately, it seems that often the information super highway is full of potholes for consumers."