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Monza, Italy - An "exceptional combined effect" of track debris and prolonged usage caused Sebastian Vettel and Nico Rosberg's high-speed tyre blowouts at the 2015 Belgian GP, Formula 1 supplier Pirelli said on Thursday.
Pirelli said in a statement: "The events at Spa can...be put down to external factors, linked with the prolonged use of the tyres on one of the most severe tracks of the championship.
The company proposed, with the governing International Automobile Federation (FIA), a study "to optimise the way in which circuits are cleaned".
FIA CLEARS PIRELLI
The FIA said it was satisfied with the thoroughness of the investigation, and Pirelli's conclusions, and was willing to consider any further safety recommendations.
Drivers said separately that teams had been advised about tyre pressures and cambers for Monza, the fastest circuit on the calendar where cars hit speeds in excess of 360km/h.
Rosberg told reporters he was confident he would have a safe car this weekend, but others remained sceptical about the conclusions.
Lotus driver Romain Grosjean told reporters: "I don't think anyone is happy with the fact that it's a cut. Seb didn't go off track, there are kerbs and you can use them.
"I don't think it's a good explanation. On the other hand, it's very hard for Pirelli to replicate what we are asking of the tyres when they don't have a current car and some testing to develop their tyres.
"We just need to find a way that the tyres don't go off (wear excessively)".
NO 'STRUCTURAL PROBLEM'
Pirelli said microscopic analysis on tyres used at Spa found no structural problem. That finding was backed up by further laboratory tests.
The company said 13 748 slick tyres had been used since the start of 2015 without problem.
However, a total of 63 cuts were found in the tread of tyres used over the Spa weekend compared to an average of just 1.2 per event in the previous 15 grands prix.
Pirelli said: "All this indicates an anomalous amount of detritus on the track in Spa, with a consequent increased risk of encountering a foreign object."
Vettel's right rear failure was consistent with a small piece of debris cutting the tyre's structural parts, without penetrating the actual structure, which then failed due to prolonged use.
Pirelli said: "Throughout the Spa weekend...cuts caused by debris were found on the tyres of other drivers, which damaged the construction but did not cause any failures."