US investigators doubt Toyota fully understands its accelerator problems and are even less sure that new floor mats are an adequate solution, a memo to lawmakers showed.

A congressional committee will publicly grill the Japanese automaker's top management as well as federal regulators later this month over the safety crisis that has led to 8.7m vehicle recalls worldwide.

Investigators briefed lawmakers from the House Oversight and Government Reform Committee on their preliminary analysis in a memorandum critical of both Toyota and the US government's National Highway Traffic Safety Administration (NHTSA), AFP reported.

"There appears to be a growing body of evidence that neither Toyota nor NHTSA have identified all the causes of sudden unintended acceleration in Toyota vehicles," said the memo dated 5 February but released this week.

"Moreover, there is substantial evidence that remedies such as redesigned floor mats have failed to solve the problem.

"This hearing will examine the question of whether Toyota and NHTSA have failed to meet their statutory and regulatory responsibilities," the memorandum read.

Representative Henry Waxman, chairman of the House Energy and Commerce Committee told AFP he had sent letters to five insurers requesting information they had received from customers about Toyota problems.

He said the committee was also seeking details of any warnings the companies may have passed on to the NHTSA, after reports said insurers had informed the agency about a pattern of customer complaints.

The House Oversight Committee's hearing, originally scheduled for Wednesday, was postponed until 24 February as another winter storm pummeled the already snowed-under US capital.

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