United Airlines issued a nationwide ground stop for all of its mainline flights late Thursday after an emergency alarm at its Chicago operations center.
"A fire alarm sounded at our operations center, which caused employees to move to our nearby backup facility and resulted in a brief nationwide ground stop of United aircraft," the company told Fox News Digital in a statement. "Employees have returned to our primary operations center, and the ground stop has been lifted."
A United Airlines passenger plane is seen at San Francisco International Airport. (Tayfun Coskun/Anadolu via Getty Images / Getty Images)
The company said that no flights were diverted and that its regional line, United Express, was unaffected.
United Airlines has the largest fleet of any American airline, operating 1,023 aircraft, according to Business Insider.