According to officials with PCSD, Federal agencies will lead the investigation.
"Safety is our No. 1 goal", said Brad Tilden, CEO of Alaska Airlines.
"In this season of life, we enjoy exploring as much as possible, whether its a day (or so) trip to one of Alaska Airline's destinations or visiting a new area of Washington", he wrote.
SeaTac Airport was temporarily grounded amid a security threat.
There was no connection to terrorism, Ed Troyer, a spokesman for the sheriff's department, said.
Traffic arrives at Sea-Tac International Airport terminal Friday evening, August 10, 2018, in SeaTac, Wash. There were no passengers aboard.
The plane was stolen around 8:00 pm (0300 GMT Saturday) and crashed 90 minutes later, officials said.
In a video posted on YouTube last December, Russell shows luggage coming off and being loaded onto aircraft, and describes what the life of a ground service agent can entail.
The plane was chased by military aircraft, witnesses said.
Horizon President and CEO Gary Beck said crew and passengers were accounted for.
We would like to thank the authorities who have been both helpful and respectful, Alaskaair for their resources, the community, his friends and his family for their incredible support and compassion, and Jesus whose steadfast love endures.
The plane was pursued by military aircraft before it crashed on tiny Ketron Island, southwest of Tacoma, Washington. Video showed flames amidst trees on the island, which is sparsely populated and only accessible by ferry.
Russell appears to have been a ground service agent, the paper said.
The Pierce County Sheriff's Office said it was "not a terrorist incident", adding the man was local and 29.
"An airline employee conducted an unauthorized takeoff without passengers at Sea-Tac; aircraft has crashed in south Puget Sound", it read.
In a bewildering move, a Horizon Air employee stole an Alaska Airlines plane on Friday and flew it erratically until crashing into an island nearby.
29-year-old Richard Russell, a ramp agent for Horizon Air, stole one of the companies aircraft and took it for a joy ride.
On Saturday, airline and Sea-Tac representatives said Russell, who identified himself as "Rich" to air traffic controllers, was fully credentialed and had access to the cargo area where the plane was.
The fighter planes intercepted the Q400 turboprop near Joint Base Lewis-McChord, NORAD said.
"Oh man", Russell says, "Those guys will rough me up if I try and land there".
The Pierce County Sheriff's Department said preliminary information suggested that the crash was caused by the person "doing stunts in the air or lack of flying skills". They should've gone over and said "Hey, ' if nothing else, 'Can I help you?" Got a few screws loose. "Normal operations at Sea-Tac Airport have resumed".
The biggest question: How did a ground service agent - whose job includes directing planes for takeoff and gate approaches as well as baggage handling and de-icing - manage to steal a 76-seat Horizon Air turboprop and fly it for almost an hour Friday night with military jets in pursuit before plunging into a wooded island?