The Federal Bureau of Investigation has recovered the flight data recorder and "components" of the cockpit voice recorder from the wreckage of a stolen Horizon Air Bombardier Q400 that crashed outside Seattle on 10 August.
"The FBI has also located human remains among the aircraft materials," says the law enforcement agency in an investigation update.
The focus of the investigation remains centered on a 29-year-old man from Sumner, Washington named Richard Russell, it adds.
However, the FBI cautions it is still awaiting results of a review conducted by the Pierce County medical examiner's office.
Russell was reportedly the Horizon ground service agent who stole the aircraft (registration N449QX) on 10 August and died when it crashed. Horizon is a subsidiary of Alaska Air Group.
The FBI does not specify the condition of the cockpit voice or cockpit data recorders, both of which are now with the National Transportation Safety Board for processing, it says.
After working the day of the incident, the ground service agent, still in uniform, used a pushback tractor to rotate the Q400 180˚. He then taxied the aircraft to the runway and took off from Seattle Tacoma International airport without clearance at 19:32 local time, according to airline officials and air traffic control recordings.
The employee, who apparently lacked a pilot's license, flew south and west, performed various rolls during the flight and was trailed by fighter jets.
The Q400 crashed on Ketron Island in the southern Puget Sound, officials say.
Airline officials say they believe Russell was alone in the aircraft and there have been no reports of other casualties.