The New York Stock Exchange suspended trading 11:32 a.m ET Wednesday.
In a brief announcement, the exchange said it was experiencing a \"technical issue.\"
\"We will be providing further updates as soon as we can, and are doing our utmost to produce a swift resolution, communicate thoroughly and transparently, and ensure a timely and orderly market re-open,\" the NYSE said.
The Department of Homeland Security told CNN that there is \"no sign of malicious activity\" at the NYSE or with an earlier outage experienced by United Airlines.
Other exchanges, including the Nasdaq, are still operating. That means investors can still trade, just not on the NYSE.
\"The NYSE outage is most severe I've seen since Nasdaq's blackout [in 2013],\" says Tim Anderson, a managing director of MND Partners who is based on the NYSE floor.
Other glitches: The Wall Street Journal's homepage stopped functioning around the same time that the NYSE went down. The Journal was able to restore a limited homepage by 12:18 pm ET with the message \"WSJ.com is having technical difficulties. The full site will return shortly.\"
United Airlines' computer system malfunctioned Wednesday morning, but it was back up by the time NYSE had problems.
Earlier in the day Wednesday, about 200 NYSE stock symbols halted trading due to a technology glitch, including Macy's (M) and Kate Spade (KATE), but that seemed to have cleared. The exchange says there is more information to come.
\"They've been having problems all day. They seemed to have rectified it earlier and then it happened again. Now they shut it all down,\" said Joe Saluzzi, co-head of trading at Themis Trading and author of Broken Markets.
Market impact: U.S. markets were already down Wednesday. The Dow was down 175 points (about 1%) around the time of the halt. The losses have deepened.
The uncertainty comes as investors were already concerned about a huge sell-off in Chinese stocks and the ongoing crisis in Greece.
\"Given the global paradigm of what's going on in the EU, Greece, China, this is the last thing that the U.S. equity markets need,\" said Peter Kenny, chief market strategist at the Clear Pool Group, a financial technology firm. Kenny worked on the NYSE floor for 25 years.
CNNMoney's Alison Kosik also contributed to this story.