The two carriers asked Snohomish Counry for approval to serve the airport four years ago.
But after the Federal Aviation Admistration issued results of an environmental study on Friday, Horizon said it would no longer pursue the plan.
Horizon cited the recession and improvements at Sea-Tac Airport.
The report concluded there would be no significant impact on traffic, noise or air quality.
At issue now is whether a new environmental study will have to be done to make way for Allegiant.
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