Adrian Newey admits he is tormented by the death of Ayrton Senna, saying "no-one will know" what really caused the fatal accident.
Three-time World Champion Senna died following a crash during the San Marino Grand Prix in 1994 and Newey was chief designer of the Williams car that the Brazilian was driving at the time.
Newey, currently the Red Bull technical boss, and Patrick Head - who was the Williams technical director at the time - were charged with manslaughter in Italy. Prosecutors believed a poor weld on Senna's steering column had snapped, causing him to lose control of his car and crash at the Tamburello corner.
The duo were eventually acquitted, but Newey is still haunted by the accident.
"What happened that day, what caused the accident, still haunts me to this day," he told BBC Sport in an exclusive interview to be broadcast on 5 live F1 on Thursday.
He added: "The steering column failure, was it the cause, or did it happen in the accident?
"There is no doubt it was cracked. Equally, all the data, all the circuit cameras, the on-board camera from Michael Schumacher's car that was following, none of that appears to be consistent with a steering-column failure.
"The car oversteered [when the rear tried to spin] initially and Ayrton caught that and only then did it go straight.
"But the first thing that happened was oversteer, in much the same way as you will sometimes see on a superspeedway in the States - the car will lose the rear, the driver will correct, and then it will go straight and hit the outside wall, which doesn't appear to be consistent with a steering-column failure."
Senna joined Williams in 1994 after winning three titles with McLaren, but he drove only three races for them in the end and the car was nowhere up to standard.
"There was an aura about him, something that's difficult to describe. He most certainly had a presence," Newey said of Senna.
"I guess one of the things that will always haunt me is that he joined Williams because we had managed to build a decent car for the previous three years and he wanted to be in the team he thought built the best car - and unfortunately that '94 car at the start of the season wasn't a good car.
"Ayrton's raw talent and determination... he tried to carry that car and make it do things it really wasn't capable of.
"And it just seems such a shame and so unfair he was in that position. And then, of course, by the time we did get the car sorted, he wasn't with us any longer."