WASHINGTON - The president of Toyota\'s U.S. operations acknowledged to skeptical lawmakers on Tuesday that the company\'s recalls of millions of its cars may not totally solve the problem of sudden and dangerous acceleration.

We are vigilant and we continue to look for potential causes, Toyota\'s James Lentz told a congressional panel. However, he repeated his company\'s position that unexpected acceleration in some of the company\'s most popular cars and trucks was caused by one of two problems — misplaced floor mats and sticking accelerator pedals.

He insisted electronic systems connected to the gas pedal and fuel line did not contribute to the problem, drawing sharp criticism from lawmakers who said such a possibility should be further explored — and from a tearful woman driver who could not stop her runaway Lexus.

Shame on you, Toyota, Rhonda Smith, of Sevierville, Tenn., said at a congressional hearing. Then she added a second shame on you directed at federal highway safety regulators.

Texas Republican Rep. Joe Barton cautioned his colleagues early in the hearing against conducting a witch hunt and said We don\'t want to just assume automatically that Toyota has done something wrong and has tried to cover it up. But midway through Lentz\'s testimony, Barton said of Toyota\'s investigation of the problems: In my opinion, it\'s a sham.

Lentz said the company had not completely ruled out an electronics malfunction and was still investigating causes of the sudden acceleration. Still, We have not found a malfunction in the electronics of any of the cars at issue, he said.