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.

Hundreds of Alaska Airlines flights and thousands of passengers’ travel plans were disrupted over the weekend and well into Monday following a “significant IT outage” that resulted in the grounding of planes for about three hours.

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.