A technical glitch knocked out trading in all Nasdaq Stock Market securities for three hours Thursday afternoon, an unprecedented meltdown for a U.S. exchange that paralyzed a broad swath of markets and highlighted the fragility of the financial world's electronic backbone.

Nasdaq officials scrambled to figure out what happened and resume trading. They shared few of their findings with trading firms or the public during regular trading hours, sowing confusion across Wall Street and leaving many investors frustrated.

The decision to reopen trading with about 35 minutes to go before the close came after exchange officials were sure that banks and brokers had enough time to prepare for securities to trade again, people familiar with the discussions said. Some hiccups persisted after Nasdaq reopened trading, though Nasdaq told traders that the markets closed normally Thursday.

"Our systems, and the industry's, have to get to a higher level of robustness," said Robert Greifeld, chief executive of Nasdaq parent Nasdaq OMX Group Inc., in an interview.

Nasdaq said it plans to work with other exchanges to investigate Thursday's outage, which centered on a problem with the data feed supplying U.S. markets with trade information, and supports "any necessary steps to enhance the platform."