Alaska Airlines resumed operations early Monday after grounding all of its planes for about three hours. An IT outage late Sunday forced the airline to halt its entire fleet, as well as planes from subsidiary Horizon Air, for the second time in just over a year, per Reuters. About three hours later, the airline said operations had resumed, but that "it will take some time to get our overall operations back to normal," per the New York Times. It did not explain what caused the outage. The issue follows a string of hacks affecting airline IT systems, including last month's attack against Alaska Air Group-owned Hawaiian Airlines.