The Pierce County Sheriff's Department in Washington state added that it was "not a terrorist incident" and that the 29-year-old man worked for Seattle-based Alaska Airlines.
The sheriff first described the man as a mechanic. But the airline later said he was a ground services agent, a position that entails directing planes to gates, handling luggage and de-icing planes.
He stole the Bombardier Q400 twin-engine turboprop plane, which typically seats about 70, around 8 pm (0300 GMT Saturday) while it was parked in a maintenance spot at the airport, the company said.
The sheriff's department said early information suggested that the plane went down because the pilot was "doing stunts" or due to a "lack of flying skills."
"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," Alaska Airlines said.
Footage broadcast by local station KIRO-7 showed the charred and smoking remnants of the aircraft on the small and sparsely populated Ketron island in the south Puget Sound outside Seattle.
Fighter jets were seen flying near the plane prior to the crash, with Washington Governor Jay Inslee later praising the pilots of the Air National Guard for scrambling to the situation.
The sheriff's department said the fighter jets played no role in the stolen turboprop's crash.
Alaska Airlines has grown in recent years from a regional carrier to operating routes across the country.