Pirelli F1 tyre boss Mario Isola has revealed Lewis Hamilton was "lucky" to avoid the same tyre failure fate as suffered by Max Verstappen and Lance Stroll.
Following preliminary investigations, Isola has confirmed the left-rear tyre issues that accounted for the Red Bull and Aston Martin drivers during the Azerbaijan Grand Prix were not tyre-wear related, but most likely due to debris.
Pirelli will now conduct a more thorough investigation at its base in Milan, including on the hard tyre of Hamilton as a six-to-seven centimetre cut was discovered, also on the left rear.
Isola said: "Looking at the [hard] tyres used in the second stint, for most of the drivers we couldn't find any evidence. I believe I can exclude that the failures were due to tyre wear.
"We found a cut on the inside shoulder on the rear-left tyre used by Lewis Hamilton from the same stint.
"The cut was quite deep and big, probably six, seven centimetres, but an odd cut in the construction so the tyre was still in one piece. Just the tread is cut.
"When there was the red flag and Lewis came into the pitlane and changed his set of tyres we were able to find the cut.
"The rear-left tyre is not the most-stressed tyre in Baku because talking about the tyres it's the rear-right.
"Another sign is that there was no sign or warning from the teams so we have to receive the telemetry from them.
"What they've told us is there was no warning, no vibration, nothing to think that there could be something in the tyres."
Asked whether Hamilton was fortunate to avoid a puncture of his own, Isola added: "The cut was not deep enough to cut the construction. He was lucky because of that.
"In the case of Max and Lance, there was an air loss, and therefore a failure."
Verstappen and Stroll incidents likely due to debris
Stroll's hard-tyre failure occurred on lap 30 as he hurtled along the main straight, sending him into a hard shunt nose-first into a concrete wall.
Verstappen's incident occurred five laps from the finish on hard tyres that were 35 laps old, again the straight at 200mph. The Dutch driver was leading the race at the time.
Pirelli had ascertained pre-race the hard tyre was good for 40 laps, with the likes of McLaren's Lando Norris doing that number when the race was stopped, while Ferrari duo Charles Leclerc and Carlos Sainz had done 38 and 37 respectively.
"I don't want to give any preliminary conclusion but it seems it is a cut due to debris," said Isola with regard to both issues.
"That is because it is not the most-stressed tyre, we have evidence of a cut in the same position, and both the accidents happened more or less on the same part of the circuit and a few laps difference [in terms of tyre age].
"We had other cars on the same number of laps, same tyres without any issue, so the preliminary investigation is that it is probably due to an external factor, debris, kerb or whatever.
"I don't want to jump to a conclusion now because now the plan is to conduct a thorough investigation and then report to the FIA and the teams before Paul Ricard [the French GP], which is a priority."