Russell was the only person aboard, and did not survive, the Pierce County Sheriff's Department said.
Video showed the Horizon Air Q400 doing large loops and other unsafe maneuvers as the sun set on the Puget Sound.
The crash sparked a fire in the dense forest.
Eckrote said the plane went down in an area covered with heavy brush and trees.
"An airline employee conducted an unauthorized takeoff without permission at Sea-Tac; aircraft has crashed in south Puget Sound", the tweet read. It normally carries 76 passengers.
"We are going to be thorough, which means taking the time needed to scour the area, delve into the background of the individual believed responsible, and review every aspect of this incident with all appropriate public & private partners", the Federal Bureau of Investigation said in a statement. Confirmed info. this is a single suicide male. We know who he is.
The 29-year-old man, who has not been officially identified, was suicidal and appeared to have acted alone, according to authorities.
The conversation between the man, addressed as Rich/Richard, and the air-traffic controller right before the Horizon Air Q400 plane crash, was released by Aviation journalist Jon Ostrower on Twitter.
"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.
"The responding fighter pilots flew alongside the aircraft and were ready to do whatever was needed to protect us, but in the end the man flying the stolen plane crashed", Inslee said.
A Horizon Air ground service agent got into a Bombardier Q400 turboprop aircraft on Friday night in a maintenance area at Seattle-Tacoma International Airport and took off, Horizon sister carrier Alaska Airline said.
An airline employee stole an airplane at Seattle airport today and was chased by two military jets before crashing on a nearby island.
The fighter jets flew at supersonic speed, triggering a boom first taken to be an explosion, as they raced to intercept the plane. "I started at 30 something..." "I thought this is really odd. Kept the video rolling". "I couldn't believe he recovered". "Most terrorists don't do loops over the water".
Normal operations at the airport were interrupted briefly, the airport said. "And then made his way down toward the island". He briefly turned away, then turned back and saw the explosion as the plane crashed.
White House Press Secretary Sarah Huckabee Sanders said this morning that President Donald Trump is "monitoring the situation". He's now at his New Jersey golf club.
"Hey, pilot guy", Rich asks. "Now let's land that airplane safely and don't hurt anybody on the ground", the helping pilot radios.
"Oh man", "Rich" answers. "You want me to ride the brakes in the cockpit" or 'Hey, what are you doing?' Nobody did that", said Kelly. "I would hope it is for a guy like me", he said.
"The fact that all of that happened without even being noticed by anyone on the ground service crew, that is just phenomenal to me", said CNN analyst and former FAA safety inspector David Soucie. "That's McChord" (meaning the military airfield at Joint Base Lewis-McChord).
"I wouldn't want to do that".
Officials said the man had been a Horizon employee for three-and-a-half years and had clearance to be among aircraft, but to their knowledge, he was not a licensed pilot.