The U.K.’s top forensic pathologist has said that Princess Diana died due to a tiny tear in a vein in one of her lungs, and would almost certainly have survived the 1997 Paris car crash that killed her had she been wearing a seat belt or had the car been traveling at a slightly slower speed.

Dr Richard Shepherd, who has performed over 23,000 post-mortems, examined the evidence as part of a British police inquiry into her death called Operation Paget. He reviews his findings in a new book, Unnatural Causes, excerpted in The Daily Mail.

According to Dr Shepherd, Diana “actually suffered just a few broken bones and a small chest injury, but this included a tiny tear in a vein in one of her lungs.”

Although her boyfriend, Dodi Fayed, who was seated next to her in the back of the car, was killed instantly, Diana was not because her bodyguard Trevor Rees-Jones, who was sitting in front of her, was, unusually, wearing a safety belt.

“Bodyguards don’t usually wear seat belts as they restrict movement, but evidently Rees-Jones, maybe because he was alarmed by Henri Paul’s driving, or maybe because he realized an impact was likely, put on his belt at the last minute,” Shepherd writes in The Mail.

“Belts are designed to give gradually while they restrain. So he was held by the belt and partially padded by the car’s airbag, which by now had inflated, as Diana’s body catapulted forward from the back seat. She was much lighter than Dodi and Rees-Jones’s belt would have absorbed some of the extra force. This slightly lessened the energy of the impact for her.

“She actually suffered just a few broken bones and a small chest injury—but this included a tiny tear in a vein in one of her lungs.

“To the ambulance services, she initially seemed injured but stable, particularly as she was able to communicate. While everyone focused on Rees-Jones, however, the vein was slowly bleeding into her chest.

“In the ambulance, she gradually lost consciousness. When she suffered a cardiac arrest, every effort was made to resuscitate her and in hospital she went into surgery, where they did identify the problem and attempted to repair the vein. But, sadly, by then it was too late.”

Shepherd said that Diana’s injury was “very small” just “in the wrong place”, adding that there were multiple ‘if onlys’ that could have prevented her death.

“If only she had hit the seat in front at a slightly different angle. If only she had been thrown forward 10mph more slowly. If only she had been put in an ambulance immediately. But the biggest if only, in Diana’s case, was within her own control.

“If only she had been wearing a seat belt. Had she been restrained, she would probably have appeared in public two days later with a black eye, perhaps a bit breathless from the fractured ribs and with a broken arm in a sling.”