The Federal Bureau of Investigation is now interviewing airport and airline workers as part of its inquiry into the theft and subsequent crash of a Horizon Air Bombardier Q400 turboprop on 10 August.
"The FBI is still in the process of interviewing relevant airline and airport personnel and collecting all footage and communications related to the incident," the agency says in a short investigation update. "We continue to identify additional individuals with information about the incident and the subject who piloted the plane."
The agency, which is heading the inquiry, has also confirmed that the person on the aircraft at the time of the accident was Richard Russell, though investigators have not yet officially concluded the cause of Russell's death.
Russell was a Horizon ground service agent, officials have said.
"The FBI is still awaiting results related to the cause of death. The FBI has no reason to believe anyone else was in the aircraft during Friday evening's flight," the agency says.
Russell allegedly stole a Horizon Q400 (registration N449QX) from a cargo area at Seattle Tacoma International airport the evening of 10 August. Horizon operates regional aircraft for its parent company Alaska Air Group.
He took off without clearance, performed aerobatic manoeuvres and died when the aircraft crashed on an island in the Puget Sound after a roughly 75min flight, officials have said.
"The FBI is working to establish the timeline of the incident and the motive of the individual believed responsible," the agency says. "The purpose of any FBI investigation is to determine if a federal crime has occurred, identify perpetrators and collect evidence."