Alaska Airlines has revealed that an "unexpected failure" of a critical piece of hardware caused an IT outage that led to its entire fleet being grounded.

Alaska Airlines and its regional subsidiary Horizon Air requested a ground stop to all flights Sunday night due to a computer system outage. The request to federal authorities kept Alaska and Horizon flights out of the air until the FAA said the stop was lifted after 2 a.m. ET. Monday.

Alaska Airlines resumed operations late Sunday night after a major IT outage grounded all flights for three hours