UPDATE: As of 20:14 (8:14pm) EST Thursday, the United Airlines ground stop has been lifted. Flights will slowly begin resuming, although I expect this to take a while.
United Airlines flights were grounded nationwide Thursday evening due to a fire alarm at the operations center in Chicago. With initially no timetable for the resumption of flights, planes on the ground were sitting on taxiways and ramps awaiting more information.
Passengers can expect residual delays as the airline tries to resume operations.
Here was the ATCSCC advisory:

The nationwide stop included all United mainline flights. Regionals were still moving; I observed several Skywest flights still on the move.
Here’s a look at Chicago O’Hare International Airport during the ground stop, where plenty of United flights could be seen parked on the taxiway. Many of these were shut down, as I heard from ATC.

The ATCSCC advisory attached a couple of photos above the cites “fire alarm” for the reason of this nationwide stop. A Reddit user onboard a United flight shared comments from the captain:
This user claimed an actual fire at the operations center, but I can only confirm a fire alarm incident for now.
This post will be updated as we receive more information.
Featured image by the author.