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One witness who might have answered that question, injured bodyguard Trevor Rees-Jones, 29, still has no memory of the final moments before the crash. Following his second interrogation last week, the former paratrooper was helicoptered back to Britain to continue his convalescence in London. French investigators may go there to question him again in a couple of weeks. But experts say there is only a slight chance that he will regain his memory of the critical instants. In the absence of his firsthand account, investigators can only sort through the physical evidence and more than 1,000 pages of testimony, much of it contradictory, to seek the precise cause of the accident. Based on interviews with eyewitnesses, experts, lawyers and sources close to the investigation, TIME has put together an exclusive account of what is known at this point about the key questions facing the French magistrates.
THE SECOND CAR. At least four eyewitnesses have described a slow-moving car driving ahead of the Mercedes in the right lane of the express road before it entered the tunnel at 12:25 a.m. on Aug. 31. Two off-duty chauffeurs standing near the tunnel entrance heard the roar of the motor as the Mercedes downshifted and accelerated. Directly in front of the speeding vehicle, they said, was a dark-colored sedan moving at normal speed. (The speed limit in the tunnel is 30 m.p.h.) They saw the Mercedes swerve into the left lane in an attempt to pass the car. Once the two vehicles entered the tunnel, these witnesses lost sight of them. But they immediately heard a loud crash followed by the droning of an automobile horn, caused by the weight of the driver's body on the wheel.
Meanwhile, a man and woman driving through the tunnel in the opposite, eastbound direction also reported seeing a dark car in front of the Mercedes. There was a sudden screech of brakes. The man heard a "small" impact, then saw the Mercedes skid directly into a support pillar in the middle of the tunnel, leading him to think the car had sideswiped the other vehicle and lost control.
The theory of a collision with a second car is supported by evidence collected just inside the tunnel entrance on the night of the accident. In addition to the Fiat Uno taillight fragments, investigators found pieces of the Mercedes' headlight and of the plastic housing of its right-rearview mirror. This debris was found near a 62-ft.-long skid mark that swerves from the right into the left lane. A short distance beyond that is the beginning of a 105-ft.-long skid mark that leads directly into the 13th pillar. All of which tends to support the theory of an initial collision followed by a loss of control.
From the earliest days of the investigation, says a judicial source, the idea of such a collision has been a "fundamental" scenario. Investigators treated the theory with caution, however, reasoning that a small, flimsy car such as a Fiat Uno would have been sent flying by a Mercedes going an estimated 70 to 90 m.p.h. After all, they said, the Fiat debris could have been left on the road by an earlier accident.