Ferrari have responded angrily after Red Bull driver Max Verstappen suggested the Italian team's poor performance at the U.S. Grand Prix was linked to Formula One's governing body closing a potential engine loophole.
Ferrari had racked up six straight pole positions, and shown impressive straight line speed, before Mercedes were back on top in the race in Austin with Valtteri Bottas winning from the front of the grid.
Ferrari were off the pace on Sunday while Bottas led team mate Lewis Hamilton, who clinched his sixth world title, in a one-two with Verstappen third.
Formula One's governing body issued a technical directive before the race in response to a Red Bull query, ruling out a sophisticated way of increasing the fuel flow to boost performance.
Asked afterwards in a news conference whether he was surprised by Ferrari's lack of performance and could offer an explanation, third-placed Verstappen suggested they had been bending the rules.