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 flights were back late Sunday night after an IT outage forced the company to ground all flights for three hours. From 8 p.m. until 11 p.m. PST, Alaska Airlines planes were on hold. The airline did not reveal the nature of the outage.

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.