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"We’re experiencing a technical issue that we’re working to resolve as quickly as possible," the NYSE tweeted shortly after the problem surfaced this morning. "We're doing our utmost to produce a swift resolution & will be providing further updates as soon as we can."

 Late in the afternoon, after trading resumed and the NYSE closed for a loss of nearly 1.5 percent for the day, NYSE Group President Tom Farley told Bloomberg TV that he decided to suspend trading because customers weren't seeing the kind of behavior they would expect to see.

"My first concern was do no harm during the day," Farley said. "Those stocks continue to trade elsewhere. Get the problem fixed. And get it back up and running for the close. We chose the least disruptive option for customers."

"I don't think the Greece situation is a focus in the markets beyond the short term," Tim Ghriskey, chief investment officer of Solaris Group in Bedford Hills, New York, told Reuters. Chinese markets are down 30 percent in the last three weeks, the news service reports. “This is really about China where the selloff continues unabated despite efforts by People’s Bank of China to halt this.”

By the time the U.S. markets closed at 4:30 — just 80 minutes after trading resumed — the Dow closed down more than 261 points, off 1.47 percent for the day.

The Nasdaq and the S&P felt pain too: the Nasdaq fell nearly 88 points and was down 1.75 percent while the S&P dropped nearly 35 points, a 1.66 decline.

One stock in particular got hit: Intercontinental Exchange, Inc. (NYSE: ICE), the owner of the NYSE, fell nearly 2.4 percent for the day but saw an uptick in afterhours trading.

As for what caused the problem in the first place, the NYSE's Farley said it originated in a configuration issue. "It likely had to do with an upgrade, but that is premature, and it's something that will come about as part of a full analysis of the situation," Farley said.