Another computer glitch is sowing confusion on Wall Street.
Trading in the Nasdaq, a major stock exchange dominated by the biggest names in technology, was halted shortly after midday Thursday due to an unknown technical glitch. Nearly two hours later, trading was still frozen. Other exchanges were trading normally.
The disruption sent brokers scurrying to figure out what went wrong and raised new questions about the pitfalls of computer-driven stock trading.
Thursday's Nasdaq freeze echoed earlier stock market snafus, such the sudden plunge in stocks in May 2010 that came to be known as the ``flash crash'' and the glitch-plagued initial public offering of Facebook last year.
The exchange sent out an alert to traders saying that trading was being halted until further notice because of problems with a quote dissemination system. Nasdaq said it wouldn't be canceling any open orders, but that customers could cancel orders if they wanted to.
Nasdaq sent out an update at 1 p.m. EDT (1700 GMT) saying trading would reopen "at a time to be determined.''
The Nasdaq composite index was stuck at 3,631.17, up 31.38 for the day. The index is up 20 percent so far this year.
Securities and Exchange Commission spokesman John Nester said: "We are monitoring the situation and are in close contact with the exchanges.''
The days of stock brokers in colorful jackets, roaming the floor of the stock exchange, are fading. Now, powerful computer programs dominate trading by sifting through reams of data and executing trades in fractions of a second. That makes trading faster and, arguably, more efficient. But it also introduces more possibilities for errors that can jolt the entire market.