The UK’s most experienced forensic pathologist, Dr. Richard Shepherd, has revealed that Princess Diana’s life could have been saved by one simple action: fastening her seatbelt.

Dr. Shepherd, who has performed more than 23,000 post-mortems during his career, is widely recognized for his work on some of the world’s most high-profile cases — from the death of Stephen Lawrence and the 7/7 London bombings to victims of 9/11. Yet, decades later, the case he is asked about most often remains the tragic death of Princess Diana in 1997.

The Princess of Wales was just 36 when she died following a car crash in Paris. She was traveling in the back of a Mercedes-Benz S280 alongside Egyptian film producer Dodi Fayed, driven by Henri Paul, and accompanied by bodyguard Trevor Rees-Jones. Paul, who was later found to be intoxicated, lost control of the vehicle while trying to escape pursuing paparazzi. Both he and Fayed were killed instantly.

Diana, however, initially survived the crash. She was conscious, talking, and appeared less badly injured than the others. But in surgery, doctors discovered a small tear to a pulmonary vein — a rare injury that ultimately caused her to go into cardiac arrest.

“It really was an exceedingly unusual injury,” Dr. Shepherd told Tyla. “She was alive, she was talking, and when they got to the car they thought she was the least injured person inside. But it was one of those insidious injuries that finally caught up with her.”