Harrowing audio revealed the pilot joking with the controllers who begged him to land the 76-seater plane before it nosedived and burst into a fireball 25 miles away.
Mr Waldron said the plane at its lowest point was no more than 100ft (30.5m) above the water.
Preliminary information suggests the crash occurred because the 29-year-old man was "doing stunts in air or lack of flying skills", the Pierce County Sheriff's Department said.
Russell, 29, a local ground service agent who assisted baggage handlers, has been identified by media as the man who operated the unauthorized flight on August 10.
The local sheriff's department confirmed that Russell appeared to be suicidal and that the unauthorized flight was not a terrorist incident.
It was unclear where he got the skills to perform such manoeuvres, authorities said.
An airline employee who stole a Horizon Air passenger plane from a Seattle airport had legitimate access to be near planes.
NORAD said the fighters did not fire on the stolen airplane.
A witness on the ground, Bryan Sichley, said he was at Chambers Bay in University Place and saw a plane nose dive after being chased by two fighter jets. "Last night's event is going to push us to learn what we can from this tragedy so that we can ensure this does not happen again at Alaska Air Group or at any other airline".
He passed a series of background checks to gain his position, as he did not have any criminal convictions. "A childhood friend remarked that Beebo was loved by everyone because he was kind and gentle to each person he met", Mathews said at the news conference.
The plane had been parked in a maintenance area and was not scheduled for a passenger flight, Tilden said.
According to the mercurynews.com, witnesses reported seeing the Bombardier Q400, a turboprop aircraft, perform a loop-the-loop and almost slam into Chambers Bay, all while being pursued by fighter jets.
Investigators are baffled as to how an unnamed mechanic stole a propeller airplane at Seattle-Tacoma International Airport on Friday and crashed it into a nearby island, New York Post reports. "Pilots kept plane out of harm's way and people on ground safe", the sheriff said on Twitter. "The situation was not their fault but I think they could have given a little more guidance or announced to the plane this is what's going on instead of leaving us all to fend for ourselves", said Leddy.
"Now we don't really know what happened, but hopefully everyone's OK".
Officials said Russell was a 3.5-year Horizon employee and had clearance to be among aircraft. It did not elaborate.
Gary Howell, a Wasilla High track and field coach, said he remembered Russell as a amusing guy who also had a smile and a joke and was a great team player.
"Could you start a left turn and we'll take you down to the SE, please", the air-traffic controller then asked.
"I don't need that much help. I've played some video games before", Russell responds.
The status of the pilot following the crash is now unknown.
"Congratulations. You did that. I don't know what the burnage is like on takeoff, but it burned quite a bit faster than I expected".
The two F-15C aircraft scrambled from Portland didn't fire at the plane, authorities said.
The FBI will lead the investigation, said Eckrote, the NTSB regional chief.
Sea-Tac Airport has issued its official response to the hijacking incident on Friday night, assuring the public that there is no further danger.