Race officials found a piece of heavy metal from Kimi Raikkonen's car at Turn 13 at Spa, just before the point where Sebastian Vettel and Fernando Alonso had punctures.
Andrew Benson's view from Spa
"I've seen the piece of metal retrieved from the kerbs at Turn 13. It's a curved piece broken off the skidplate of Kimi Raikkonen's Lotus. The two edges match puncture marks on Fernando Alonso's failed tyre, and the width of a part of it matches the cut in Sebastian Vettel's.
"Raikkonen had an off there in second practice and Alonso and Vettel both ran wide at the same place shortly afterwards. Gary Anderson describes the discovery as 'good detective work', but adds: 'Tyres need to be durable enough to withstand that and at the moment I don't think they are durable enough.'"
But Red Bull's Mark Webber said the drivers would accept the explanation.
Ferrari development driver Pedro De La Rosa, the chairman of the Grand Prix Drivers' Association, said he was "very happy" that Pirelli and governing body the FIA had found what appeared to be the reason for the failures.
"We only asked to know what it was," he told BBC Sport. "If it had been the first time we'd seen it we would have been relaxed, but after all the problems this year we were definitely concerned."
Pirelli changed the design of the tyres following a series of failures during the British Grand Prix.
The new tyres are of a more robust construction and were designed with the extreme demands of Belgium's Spa-Francorchamps track in mind.
The failures on Vettel's and Alonso's cars happened in the same area of the track, between Turns 13 and 14, the Fagnes and Stavelot corners.
F1: Dramatic tyre failures hit British Grand Prix
Pirelli motorsport director Paul Hembery and FIA race director Charlie Whiting went out on to the track on Friday to investigate what might have happened after Hembery said initial indications were the tyres had been punctured by some debris.
They found a piece of tungsten - now understood to have come from Raikkonen's car - in the kerbs on the exit of the Fagnes left-hander, otherwise known as Turn 13.
The right rear tyre of the cars would run over this kerb as they exited that corner, which immediately precedes the Stavelot and Paul Frere corners where the failures on the cars of Alonso and Vettel were discovered.
The piece of metal was exactly the same size as the hole in Alonso's tyre and a similar diameter to the width of the cut running along Vettel's tyre.
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