All flights were grounded at Seattle-Tacoma international airport in the US when a passenger plane took off without permission.
Authorities said an airline employee had made “an unauthorised take-off” without passengers.
Officials said the plane later crashed into the sea. The identity and condition of the pilot are not known.
US fighter jets were reportedly scrambled when the passenger plane took off.
The plane was a Horizon Air Q400, according to a statement from Horizon’s partner airline Alaska Airlines.
The Q400 is a twin-prop aeroplane with up to 78 seats, depending on configuration.