SEATTLE — The story is as bizarre as it is tragic: An airline worker who had no business being in a cockpit somehow took off from a major airport in a turboprop passenger plane, dipped and soared in the skies above the Seattle area, and then crashed into an island on Puget Sound.

It was all over in about an hour. But the consequences of the unauthorized flight on Friday evening are monumental, aviation experts and investigators said.

The man — a ground service agent identified as Richard B. Russell, according to a law enforcement official — exposed a troubling reality of airport security in the post-9/11 era. While many visible aspects of commercial flight that affect the routines of passengers have been hardened, parts of the system that are behind the scenes, but just as important to public safety as cockpit doors and screening machines, remain vulnerable.

Mr. Russell took off around 7:30 p.m., according to the authorities. He chatted sometimes calmly and sometimes in a frenzied stream of consciousness with air traffic controllers who tried to guide him to a safe landing, as jets from the Air National Guards of Washington and Oregon flew alongside him, ready to take action. The plane came down in a fiery crash on Ketron Island, about 30 miles from the airport. No one else was believed to be on board, and officials confirmed Mr. Russell was killed.