Norad said the F15s were "working to redirect the aircraft out over the Pacific Ocean when it crashed on the southern tip of Ketron island", about 30 miles (48km) south of the airport.
A bipartisan House bill approved past year, 409 to zero, would tighten employee background checks and increase surveillance of secure areas at airports.
The local sheriff's office said the crash may have been caused by the man "doing stunts in air or lack of flying skills".
It wasn't immediately clear what training, if any, the man had in flying planes.
But concern about the mental state of aviation workers has grown in recent years, analyst say, particularly after the 2015 crash of Germanwings 9525 flight in France, when a pilot deliberately steered his plane into a mountainside, killing 144 passengers and five crew members.
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.
There was no connection to terrorism, said Ed Troyer, a spokesman for the sheriff's department.
They believe there were no passengers on board and no structures hit when the plane crashed into Ketron Island on Friday night. There were no passengers aboard. Russell used a push-back tractor to rotate the plane 180 degrees before getting in and taxiing the aircraft.
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The plane was pursued by military aircraft before it crashed on Ketron Island, southwest of Tacoma, Washington. They also will work to recover the remains of the 29-year-old Pierce County man thought to be the only person on the plane.
Tilden said the system in place is to credential and secure employees and air fields, but not planes.
Alaska Airlines on Twitter said the airplane was a Q400 airplane belonging to Horizon Air.
Per CNN, the man also had some freaky and heartbreaking radio correspondence from the cockpit before the 76-seat Horizon Air turboprop went down.
The man was a resident of Pierce County, Wash., and "acted alone", the Pierce County Sheriff's Department tweeted.
Audio recordings from the air traffic control tower at Sea-Tac feature the conversation between the tower and the man in the airplane.
The air controller tells him there is a runway just off to his right "in about a mile [1.6km]", referring to a military airfield at Joint Base Lewis-McChord.
He also says: "I've got a lot of people that care about me". They should've gone over and said "Hey, ' if nothing else, 'Can I help you?" It's going to disappoint them to hear that I did this.
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Those agents guide planes, handle bags and de-ice planes, Horizon Air says.
On Saturday morning, White House Press Secretary Sarah Sanders told media that President Donald Trump was briefed on the incident and is now "monitoring the situation" in Seattle. Trump is now at his New Jersey golf club, and Sanders commended the response effort for its "swift action" and public safety protection.
The U.S. Coast Guard sent a 45-foot vessel to the crash scene.
The sparsely populated island remained close to ferry traffic Saturday morning.
Gary Beck, CEO of Horizon Air, told reporters that "to our knowlege, he didn´t have a pilot´s license".
"It just hurts to hear someone you know, and just hear the pain in their voice", Kaelin said.
"The greatest threat we have to aviation is the insider threat", Erroll Southers, a former Federal Bureau of Investigation agent and transportation security expert, told the AP.
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Immediately give these to the banks and convert to Turkish lira and by doing this, we fight this war of independence and the future.