Trading on the New York Stock Exchange was shut down for hours on Wednesday as the exchange tried to cope with what appeared to be a technical glitch, rather than an attack.
It was the longest such suspension of trading at the exchange in recent years, although trading in the stocks listed on the NYSE was able to continue on other stock exchanges, like Nasdaq.
Trading resumed late Wednesday afternoon, almost four hours after the shutdown began, less than an hour before the 4 p.m. closing bell.
When the shutdown began, Peter Costa, a trader on the floor with Empire Executions, said that exchange employees manually canceled about 700,000 orders that were in the system.
After canceling the orders, the exchange rebooted its systems, which was expected to take about 45 minutes.
The first technical problems appeared soon after the opening bell rang at 9:30 a.m. Wednesday, when orders for several smaller stocks failed to go through, Costa and another trader on the floor said.
The initial issues only hit a small number of stocks, and the exchange appeared to have dealt with the problem, but they reappeared, on a more widespread basis, later in the morning, the traders said.
At 11:32 am, the exchange announced that it was shutting down all trading.
While the exchange said on Twitter that it decided to shut down trading to deal with the technical problems, it gave traders on the floor no warning before making the move.
"I was about to order my lunch and when I turned around the screens all went blank," Costa said.
The shutdown was especially troubling because it came only hours after United Airlines temporarily grounded all of its flights following a technical issue. Other businesses had problems with their websites.
But both United and the New York Stock Exchange were adamant that the problems were a result of internal technical problems, rather than malicious hackers.
While, there was no indication that the incidents were related, one prominent investor, Doug Kass, noted that the problems were a reminder that "in a paperless (and 'cloudy') world, investors and citizens are not likely as safe as the markets assume."
The New York Stock Exchange, which is now owned by Intercontinental Exchange, has had, like other stock exchanges, technical difficulties in the past, but the scale of the problem on Wednesday has little precedent.
Other exchanges operated by the New York Stock Exchange, such as its options exchange, continued operating normally Wednesday.
Federal regulators and law enforcement agencies are monitoring the situation but as of midday Wednesday there was no indication of anything malicious with the troubles at the NYSE, said several people briefed on the situation.
Mary Jo White, the chairwoman of the Securities and Exchange Commission, which oversees the nation's securities markets, said in a statement, "We are in contact with NYSE and are closely monitoring the situation and trading in NYSE-listed stocks."
President Barack Obama was briefed on Wednesday about the New York Stock Exchange outage, a White House aide said, adding that officials there and at the Treasury Department were monitoring the situation.
The outage at the New York Stock Exchange came on the same day that a technical problem forced United Airlines to ground all of its planes in the US for more than two hours.
The Nasdaq said on Twitter that all of its systems were operating normally.
Amid the halt in trading, the website for The Wall Street Journal was also down. The site eventually started loading, but in a pared-down capacity. "WSJ.com is having technical difficulties," a statement on the site read. "The full site will return shortly."
More information on the outage:
(1 of 3) The issue we are experiencing is an internal technical issue and is not the result of a cyber breach.— NYSE (@NYSE) July 8, 2015
(2 of 3) We chose to suspend trading on NYSE to avoid problems arising from our technical issue.— NYSE (@NYSE) July 8, 2015
(3 of 3) NYSE-listed securities continue to trade unaffected on other market centers.— NYSE (@NYSE) July 8, 2015
(1 of 2) We’re experiencing a technical issue that we’re working to resolve as quickly as possible.— NYSE (@NYSE) July 8, 2015
(2 of 2) We're doing our utmost to produce a swift resolution & will be providing further updates as soon as we can.— NYSE (@NYSE) July 8, 2015
Nasdaq systems are operating normally and are trading all symbols including Tape A (NYSE) securities.— Nasdaq (@NASDAQ) July 8, 2015
Material from The Associated Press was used in this report.