All stocks trading on the New York Stock Exchange were halted for more than three hours on Wednesday following a technical glitch.
"We're experiencing a technical issue that we're working to resolve as quickly as possible," the stock market said in a statement.
"Additional information will follow as soon as possible."
A little after 3 p.m. ET, the exchange reopened and traders saw a trickle of buy and sell orders.
While the NYSE is still not saying what exactly went wrong, the exchange said it had a technical glitch that resulted in some traders not receiving confirmation of orders they had made or received for at least 221 different stocks.
Trading was then halted in all shares as of 11:32 a.m. ET.
The exchange said the interruption was not a breach or a cyberattack. U.S. law enforcement and security officials also said they didn't see any link between the NYSE glitch and the two other outages on Wednesday. United Continental temporarily grounded all of its flights due to a "network connectivity issue" and the Wall Street Journal website was briefly inaccessible because of "technical difficulties."
Shortly after the NYSE shutdown, FBI director James Comey told the Senate intelligence committee, "I think the Wall Street Journal piece is connected to people flooding their website in response to the New York Stock Exchange to find out what's going on. In my business, you don't love coincidences, but it does appear that there is not a cyber intrusion involved."
The Toronto Stock Exchange was open throughout the NYSE outage and finished the day 212 points lower at 14,412.
Other U.S. exchanges, including the Nasdaq which lists shares in some of the same companies that the NYSE does, were operating normally.
"It's disruptive, but not wildly disruptive," said Thomas Caldwell, chairman of Caldwell Securities. "You have so many competing exchanges."
Before and after NYSE trading was halted, U.S. indexes were lower, as investors digested bleak economic news out of Greece and a Chinese stock market tumble that Beijing officials seem powerless to slow.
The Dow Jones industrial average closed down 261 points, or 1.4 per cent, to 17,515.
The Standard & Poor's 500 gave up 34 points, or 1.7 per cent, to 2,046. The Nasdaq was down 87 points, or 1.7 per cent, to 4,909.
Wednesday's outage is the largest and longest in the NYSE's recent history, dating back to the so-called Flash Crash of May 2010 when the Dow Jones Industrial Average lost hundreds of points in minutes due to a technical glitch.