After taking off from Seattle-Tacoma International Airport, the hijacked plane crashed on Ketron Island.
The FBI's Seattle field office said on Twitter it was working with local, state and federal partners, but it was "too early for us to put out details on the evolving situation".
Witnesses reported seeing the plane being chased by military aircraft near the airport.
A user on Twitter captured video of the Horizon Q400 plane that made an unauthorized takeoff from Seattle-Tacoma International Airport on Friday as it was flying.
Police and Coast Guard are now at the scene of the crash, with the Pierce County Sheriff's office tweeting the plane was stolen by a mechanic who was "doing stunts" before the crash. As of 12:30 a.m., SeaTac tweeted that normal operations had resumed.
I want to thank the Air National Guard from Washington and OR for scrambling jets to keep Washingtonians safe.
Witnesses reported seeing the plane being chased by military aircraft before it crashed near Ketron Island.
This individual who took the aircraft, who has not yet been positively identified until remains are examined, is believed to have been the only person on the plane when the plane was taken from a maintenance position at Sea-Tac.
His motive is not clear, but audio between the cockpit and air traffic control indicates the man was in emotional distress. The identity and condition of the pilot are not known.
Ketron Island is a sparsely populated island near Anderson Island, off Steilacoom, Wash. "The FAA is not a security agency, although we work closely with other government agencies on security issues". More information as we learn more'.
The plane crashed on Ketorn Island outside Seattle. "The Bombardier Q400 features 76 comfortable seats and boarding doors in both the front and rear of the plane for faster passenger loading and deplaning".
The man could be heard telling air traffic controllers that he was "just a broken guy" but was also heard joking about becoming a pilot if he landed safely.
Sea-Tac officials said the airline employee had "conducted an unauthorised takeoff without passengers".
It said that the F-15 pilots kept the "Q400 out of harm's way and people on the ground safe".