Witnesses reported seeing the plane being chased by military aircraft before it crashed.
Two F-15s fighter jets pursued the plane but were not involved in the crash, local TV station KIRO7 reported.
"Our hearts are with the family of the individual aboard, along with all of our Alaska Air and Horizon Air employees", Horizon Air Chief Operating Officer Constance von Muehlen said in a video posted on Twitter.
Kaelin said Russell "was always great to work with - we got our flights out on time, if not early, most of the time".
The freaky incident points to one of the biggest potential perils for commercial air travel - airline or airport employees causing mayhem.
Investigators would not name the employee who took the plane, but a law enforcement source identified him to KUOW as Richard B. Russell of Sumner, Washington.
Authorities say the man went through various background checks to get clearance to be in the secured area.
Military jets had been scrambled to chase the plane, according to the Times. Authorities said he took the airplane from the maintenance area and used a push back tractor to rotate it 180 degrees before takeoff.
A Horizon Air airplane was stolen from SeaTac Airport on August 10, 2018 and crashed on Ketron Island where it sparked a fire. There were no passengers aboard.
Russell worked for Horizon Airlines, a sister carrier of Alaska Airlines [ALKAIR.UL], as a ground service agent who helped baggage handlers and was part of Horizon's tow team, which moved planes around on the tarmac.
According to BuzzFeed, the Federal Bureau of Investigation and airline officials did not identify Russell, but said he joined Horizon Air in February 2015, which matched information listed on his LinkedIn page.
He said:"I wouldn't know how to land - I wasn't really planning on landing it".
The plane, a 76-seater Q400 turboprop aircraft, belonged to Alaska Airlines.
Traffic arrives at Sea-Tac International Airport terminal Friday evening, August 10, 2018, in SeaTac, Wash.
Horizon Air employee Richard Russell told an air traffic controller he was "just a broken man" minutes before dying late Friday in the Bombardier Q400 twin-engine turboprop plane, appearing to apologize for his actions.
At one point, he said: "Hey do you think if I land this successfully Alaska will give me a job as a pilot?"
"I want to thank the Air National Guard from Washington and OR for scrambling jets to keep Washingtonians safe", Inslee tweeted, adding: "Those pilots are trained for moments like tonight and showed they are ready and capable".
"Got a few screws loose, I guess", he is heard saying in the recording.
The crash sparked a fire in the dense forest.
"It is highly fragmented", she said of the plane.
Royal King told the Seattle Times he was photographing a wedding when he saw a plane and two F-15 fighter jets trailing it.
"It was unfathomable, it was something out of a movie", he said.
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