There was no indication that the incident was an act of terror, the Pierce County Sheriff's Department said on Twitter.

No passengers were onboard and normal operations have resumed at the airfield, the airport said on Twitter.

The 29-year-old pilot was suicidal and appeared to have acted alone. He was probably killed in the crash, the sheriff said.

Alaska Airlines said that it was aware of an unauthorized take-off of a Q400 twin-engine turboprop airplane operated by its sister carrier Horizon Air.

Two F-15s fighter jets pursued the plane but were not involved in the crash, KIRO7 reported.

"Told F-15s made it within a few minutes of theft of plane. Pilots kept plane out of harm's way and people on ground safe," the sheriff said on Twitter.

Video footage on social media showed a large plane flying above the Seattle area with an F-15 following it.

"Okay, this insane. A pilot on the plane in front of us just went rogue and took off on an empty plane bypassing orders from the tower," tweeted Ben Schaechter, a passenger on an airplane that was taxiing to take off before the incident.

Local media showed video of a fire from the crash on Ketron Island in Puget Sound.

The Bombardier Q400 turboprop airplane is designed for shorter-distance flights and can seat 76 passengers, Alaska Air said on its website.

REUTERS