U.S. carrier Alaska Airlines resumed operations late on Sunday after briefly grounding all of its flights for about three hours due to an IT outage, marking the second time it has halted its fleet in just over a year.

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.