The US FBI investigation of the Horizon Air Bombardier Q400 crash has concluded the airline employee who stole the aircraft at Seattle-Tacoma (Sea-Tac) International Airport deliberately crashed it into the ground.

The incident began around 7:30 p.m. Pacific Aug. 10, when a Horizon Air ground service agent with no flying experience—later identified as 28-year-old Richard Russell—was able to make an unauthorized takeoff from SeaTac and fly for more than an hour before crashing on a remote island in the Puget Sound.

Because the investigation revealed Russell, who died in the crash, had planned and executed the theft without any accomplices, there will be no criminal charges filed.

The FBI, using information from the US National Transportation Safety Board’s review of the plane’s flight data recorder and cockpit voice recorder (CVR), determined the airplane remained in control during its final descent to the ground.

Had Russell intended to avoid impact with the ground, he would have had ample time and energy to raise the plane’s nose and initiate a climb, the investigators reasoned. Instead, the plane remained in a position forward of neutral, moving further forward about six seconds prior to crashing.

The CVR did not capture any sounds from the cockpit beyond the voice communications Russell sent out over the cued microphone, which were made publicly available shortly following the crash. The pilot did not make any other phone calls or statements in the cockpit that addressed his motive.

FBI interviews with Russell’s co-workers, friends and family members—as well as a review of text messages he sent out during the incident—did not produce any new information that would suggest the theft was linked to any wider criminal activity or terrorist ideology. Investigators who reviewed Russell’s background, possible stressors and personal life were unable to identify a clear motivation for his actions.

The investigation found that Russell was properly credentialed, had access to the interior and exterior of the aircraft, and did not violate any security protocols prior to his theft of the plane. While the investigation did not reveal Russell had any formal flight training, investigators did learn he was familiar with the checklist of actions for starting an airplane and had conducted internet searches for flight instructional videos.

The US Transportation Security Administration (TSA) conducted a separate investigation into the incident and determined that Horizon Air and Sea-Tac Airport were in compliance with all safety requirements. TSA, in a statement, said it is working with industry partners and its Aviation Security Advisory Committee to examine ways to protect against insider threats to aviation.

Ben Goldstein, Ben.Goldstein@aviationweek.com