The New York Stock Exchange is back up and running as of 3:13 p.m ET Wednesday after a technical glitch shut down the exchange for 3 hours and 38 minutes.
, chose to suspend trading to avoid problems arising from the internal issue, the company said.
An expert in trading systems told Fortune that a failed systems upgrade likely forced the NYSE to shut down. Eric Scott Hunsader, head of market data firm Nanex, said there were indications that the NYSE was having problems even before the market opened Wednesday.
Traders received an alert from the exchange that warned of certain “gateways,” which traders use to connect with the NYSE, that would be shut down due to glitches. The problems would likely affect a “subset” of stocks, the exchange said.
The NYSE said the issues were fixed as of 10:37 a.m. ET, but that’s when the market started to shut down. Trading was halted shortly thereafter.
This story originally appeared on Fortune.