A lack of oxygen likely incapacitated the pilot of a Cessna Citation 560 in June 2023 that prompted the U.S. military to scramble F-16 fighter jets before the private jet crashed in Virginia killing all four occupants, a final report said on Tuesday.

The National Transportation Safety Board said it is likely the pilot and three passengers became incapacitated during the aircraft’s climb to cruise altitude.

The board said it appeared the plane was on autopilot at the time when it crossed into restricted airspace around Washington.

Maintenance records raised questions about the issues around the pressurization and environmental control system weeks before the crash that the owner had declined to address.

The jet fighters created a sonic boom over the U.S. capital region as they pursued the errant Cessna. A Cessna Citation can carry seven to 12 passengers.

The U.S. military attempted to contact the pilot, who was unresponsive, until the Cessna crashed in the mountains of Montebello, Virginia near the George Washington National Forest.

The Cessna began the day at its home airport in Melbourne, Florida, and later took off from Elizabethton Municipal Airport in Elizabethton, Tennessee. It was bound for Long Island MacArthur Airport in New York, about 50 miles (80 km) east of Manhattan.

The NTSB said the plane reached the airport and then made nearly a 180-degree turn.

The airplane was intercepted by U.S. Air Force fighter aircraft minutes before it crashed.

Pilots observed a person seated in the left cockpit seat slumped completely over into the right seat who remained motionless and unresponsive to radio transmissions, intercept flight maneuvers, and flare deployments.

Incidents involving unresponsive pilots are not unprecedented. Golfer Payne Stewart died in 1999 along with five others after the aircraft he was in flew thousands of miles with the pilot and passengers unresponsive. The plane eventually crashed in South Dakota with no survivors.

In the case of Stewart’s flight, the plane lost cabin pressure, causing the occupants to lose consciousness because of oxygen deprivation.