The problem was later traced to a computer cable connected to a mainframe computer at Nasdaq's technical center in Trumbull, Conn., a Nasdaq spokesman said. He said no further information was immediately available on the problem.

At about 11:30 a.m. EST, the composite index stopped updating, and it remained stuck at 4535.5, a constant 13.4 point decline, before resuming movement shortly after 2 p.m., with a sharp drop of nearly 75 more points to 4461.1.

In subsequent afternoon trade, the composite continued to stumble, losing 110 points, or 2.43 percent. Earlier in the day, the reading had reached an all-time intraday high of 4564.45.

Detailing the breakdown, Nasdaq spokesman Scott Peterson, said that users of the Nasdaq Workstation II, the Nasdaq's primary trading device — mainly market makers and order-entry firms — were not receiving the last prices for securities with ticker symbols beginning with the letters "A" through "J."

"Although the last sale prices were not being disseminated on (the Nasdaq Workstation II), the (updated) information was still available to the public, as the information was reaching market-data vendors" such as the Associated Press, Reuters Group Plc, Bridge Information Systems and Bloomberg LP, he said.

The glitch that froze the index was annoying the traders at Carlin Financial Group but causing them no real pain, said Mayer Offman, a partner at the New York-based stock trading firm.

The problem was, however, rattling investors and helping create uncertainty in the market, Offman said. Such woes are a reminder that computer problems can wreak havoc on an already highly volatile market, he said.

"I think it is having an effect on the market," he said. "I think people are afraid of hackers," he said. "It's not affecting trading of individual stocks, but it's causing the markets to go down."