Ralf Schumacher's heavy crash at Indianapolis was caused by a failure of the left rear tyre, and came after a similar problem for Toyota third driver Ricardo Zonta.

Because it happened at a 'safe' corner, the Brazilian's incident did not have serious consequences, but Toyota and Michelin are obviously taking a close look at the remains of both damaged tyres. It is not known how many other teams are using that particular compound/construction - the company always brings a range, depending on what its customers have chosen in the weeks before the race - but the double failure has to be a concern for all seven of the French company's runners.

"Thankfully I am feeling okay, but that was quite a big accident," said Schumacher, reflecting on the similarity between today's shunt and that in which he broke his back at the same corner inn 2004.

"Lightning is supposed not to strike twice in the same place but, on this occasion, I guess that does not apply to me. I was approaching the last corner when I felt something go wrong on the left-hand side. We will now have to investigate exactly what happened - and why."

"On both occasions, we had no early indication from the tyre pressure sensors, so it was a fast deflation," revealed Toyota chief engineer Dieter Gass, "We don't know the reason why at the moment, so we will now set about analysing the data to understand exactly what happened. Maybe there is a point on the track where we are running over and cutting our tyres, because it seems to occur on the outside tyre.

"We made sure that we had no problem on the car and that the suspension was okay. Then we talked to our partner Michelin and agreed that it would be safe enough to go out again. But we subsequently found a third cut tyre on Ricardo's car, so we stopped running early. It all means that we are missing a lot of data for the weekend, but we must now analyse what is happening and carry on from there.

"Ralf's car is not as badly damaged as we feared and it looks like we can use the engine again tomorrow. But the main thing is that all our drivers are okay."

The fact that both failures occurred in the same team may indicate that low pressures were a factor, and it's thought that teams have been advised to raise the minimum levels they operate with, especially at the rear. However, there could be problems on race day when a safety car period could cause pressures to drop off.

"It's a little bit worrying to see that happen, especially to him twice now," said BAR Honda's Jenson Button, "I should think he's pretty annoyed! The good thing is they've got the 'soft wall' there all the way through the corner. It was a pretty big impact, but he was able to get out and walk away.

"There will be a lot of discussions between both tyre manufacturers and all the teams I'm sure, because we don't want that happening again."

Although he passed a quick FIA medical check-up after the crash, Schumacher later complained of problems with blurred vision, and was taken to an Indianapolis hospital. The official line was that he might have debris in the eye.

If he is not passed fit for Saturday, or gets out of the car after trying it in the morning, 'supersub' Zonta will be able to step in and take over at any point before qualifying.

"The track has evolved since this morning, as the F1 cars and Infiniti Pro Series runners have laid down more rubber, but parts of it remain extremely abrasive - particularly the recently-resurfaced banked section and pit straight, which are common to both the road course and the oval," said Michelin's Pierre Dupasquier.

"It is clear that the two Toyotas suffered rear left problems. No cause has so far been definitely established, and we are continuing our investigations. We are pleased to note neither driver suffered any major injuries. There were 17 Michelin cars on the circuit today and we did not have any widespread problems. Our engineers will continue to work very closely with those of Toyota while the matter is investigated."