A Horizon Air employee intentionally crashed a stolen turboprop plane in August but there was no clear motive for his actions, federal investigators said Friday, bringing to a close a probe of an incident that prompted fresh scrutiny of airport security.
Richard Russell, a 28-year-old Horizon Air ground services worker, stole the empty plane on Aug. 10 at Seattle-Tacoma International Airport before crashing it on a sparsely inhabited island nearby. Mr. Russell died in the crash.
The Federal Bureau of Investigation said its probe didn’t find any evidence suggesting “wider criminal activity or terrorist ideology” in the incident that sparked widespread alarm and interception by military fighter jets. “Although investigators received information regarding Russell’s background, possible stressors, and personal life, no element provided a clear motivation for Russell’s actions,” the agency said in a two-page statement on Friday.
The FBI added that it found no co-conspirators and wouldn’t be pursuing federal charges in the case.
A representative for the Russell family couldn’t immediately be reached for comment.
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The incident has spurred reviews of airport and airline security. The federal Transportation Security Administration said its own investigation into the incident found Horizon, which is owned by Alaska Air Group Inc., and the airport were in compliance with all security requirements. But the TSA said it is working with the industry and an advisory committee to examine ways to boost airport security.
The FBI said its investigation included interviews with fellow workers, friends, and relatives, as well as a review of text messages exchanged with Mr. Russell during the incident.
A review of the Horizon Q400 plane’s flight-data recorder by the National Transportation Safety Board showed the plane underwent a “significant sideslip,” during the final minute of the flight, the FBI said, suggesting it was slipping through air sideways. But the plane appeared to have remained in control and the “final descent to the ground appears to have been intentional,” the FBI said.
In the flight’s final moments, Mr. Russell had time to raise the plane’s nose, climb and avoid impact with the ground, investigators said.
The plane’s cockpit voice recorder didn’t capture any significant sounds beyond the air-traffic control communications that have been available publicly since the incident, the FBI said. Mr. Russell didn’t make any phone calls while in the cockpit or otherwise say anything that addressed his motive.
An FBI spokeswoman said investigators found no suicide note.
Ryann Sale, an investigator at the Pierce County Medical Examiner’s office in Washington, said Mr. Russell’s death was ruled a suicide caused by “multiple traumatic injuries,” but declined to comment further.
The FBI spokeswoman said the medical examiner’s office performed fingerprint analysis to conclude that the remains at the crash belonged to Mr. Russell. Asked if investigators were able to determine whether Mr. Russell was under the influence of alcohol or drugs, she added: “There was insufficient material to facilitate a toxicology screening.”
Until he stole the plane, Mr. Russell didn’t appear to violate any security measures, according to the FBI. Investigators also said Mr. Russell was properly credentialed and had access to exterior and interior of aircraft as part of his job, which involved moving planes around the airport and using their power systems. Mr. Russell didn’t have any formal flight training, but the FBI said investigators learned Mr. Russell was familiar with a checklist of actions pilots undertake for starting aircraft.
Mr. Russell also performed internet searches for flight instruction videos, but the FBI said investigators “did not uncover any conclusive evidence to suggest further, informal flight training.”
Alaska Air said it appreciated the FBI’s thorough investigation. Gary Beck, Horizon Air chief executive, said: “This incident was a very difficult moment for us and many others.”
Write to Andrew Tangel at Andrew.Tangel@wsj.com