Alaska Airlines has resumed operations following an undisclosed technical error affecting its entire system for approximately three hours.

Alaska Airlines operates 238 Boeing aircraft, 87 Embraer planes, and 45 additional planes through Horizon Air. The Seattle-based company is the fifth-largest airline in the U.S., serving more than 44 million passengers annually.

Alaska Airlines warned of more flight disruptions on Monday after an unexpected failure of a critical piece of hardware at its data centers late Sunday forced it to halt all flights for about three hours.