Kimi Raikkonen says Ferrari has yet to determine why the rear wheel nut on his car was not fitted correctly following his third and final fateful stop in Sunday's United States Grand Prix.

Raikkonen was on an aggressive three-stop strategy when he pitted on lap 38. However, as he was exciting the pits he was told to stop and he then had to reverse back into the pit lane to retire. The Scuderia was subsequently fined 5000 Euros by the FIA for an unsafe release.

"Obviously we had some issues with the pit-stop and the tyre was never tightened 100 per cent," Raikkonen said.

"It was attached somehow because it didn't come off and the nut was on when I stopped. But we noticed something was going on and I slowed down straight afterwards because I saw some sparks. But I don't know the reason why it never got done 100 per cent," he added.

"Now we have to understand what happened."

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Meanwhile, Raikkonen added that he was happy with his car until he switched to the soft tyre on lap 8.

"The car was behaving pretty good, but from the second set, for whatever reason, they were new tyres, but they never really had the grip like I expected," he explained.

"I could follow the other guys in front but in my view we were just struggling for overall grip on that set and when we put the supersofts back on the car [on lap 24 it] was again very good.

"But obviously none of those things matter because in the end we didn't finish the race. It is one of those weekends. We had decent speed in the race, but didn't make anything of it."

United States Grand Prix - Race results

Asked if he felt the strategy was right and if the Scuderia should have pitted him under the Virtual Safety Car period, he replied: "We never found out and in the end nothing changed because we never finished the race.

"There is always if. But the fact is we never finished," he underlined.

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