Toyota's head of US operations has admitted that recalling millions of vehicles to fix problems with their accelerators may "not totally" solve the issue.
James Lentz told a US congressional panel: "We are vigilant and we continue to look for potential causes."
However, he insisted that floor mats and sticking accelerator pedals were behind the sudden acceleration in some of Toyota's popular cars and trucks
Mr Lentz added that electronic systems connected to the accelerator pedal and fuel line did not cause the problem.
Politicians have criticised him, saying the Japanese carmaker should further investigate into such a possibility.
The US Congress had previously heard from a tearful woman motorist who could not stop her runaway Lexus, and doubted the car's electronic system for the unexpected acceleration.
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