Toyota Motor Corp has failed to support statements of top executives that the automaker has rigorously evaluated electronic throttles in its vehicles, Democratic leaders of a congressional committee said on Friday.

The assertion by Henry Waxman and Bart Stupak, chairmen of the House of Representatives Energy and Commerce Committee and its investigative subcommittee, respectively, added to the fallout from Toyota's safety and recall crisis that has shaken the automaker's reputation for quality.

Toyota has recalled more than 6 million cars and trucks in the United States since October for equipment and mechanical problems related to unintended acceleration.

But questions about possible glitches in throttle software and whether that is behind at least some cases of unwanted acceleration in Toyota and Lexus vehicles are central to ongoing congressional and regulatory investigations.

Those questions were magnified this week by regulators, who said they were investigating more than 60 complaints from motorists alleging that recall fixes had not solved their problems with unintended acceleration.

Edolphus Towns, chairman of the House Oversight and Government Reform Committee, late on Friday asked the National Highway Traffic Safety Administration for monthly reports on post-recall complaints and what the agency and Toyota are doing to resolve the matter.

NHTSA and Toyota are investigating the complaints. Toyota said on Thursday a partial review of reports found no evidence of problems with the fixes or the electronic throttle systems.

The automaker said it has fixed more than 1 million cars and trucks since recalling floormats that can jam the accelerator in October 2009 and gas pedals that do not spring back as designed in January.

The Energy and Commerce Committee, which will hold its second hearing on Toyota on March 11, said in a letter to Toyota U.S. sales chief Jim Lentz that thousands of documents turned over to the panel in the past month have not sufficiently supported statements by executives that exhaustive testing has found no throttle problems.

Despite our repeated requests, the record before the committee is most notable for what is missing -- the absence of documents showing that Toyota has systematically investigated the possibility of electronic defects that could cause sudden acceleration, the Democratic lawmakers said.

The letter said some documents contain information that could be used in planning a rigorous study. But not one of them suggested that a rigorous study had taken place.

Toyota said in a statement on Friday that it is cooperating with the committee and is providing more information on an independent study of its throttle systems by outside consultant Exponent Inc as well as the results of its own testing.

Waxman and Stupak in their letter questioned the preliminary results of Exponent which found no problems with the carmaker's throttles.

NHTSA has also found no problems over the years. But committees in both houses of Congress have questioned whether the agency conducted thorough investigations and whether it has adequate resources to do the job now. Regulators have said they may seek outside help on the Toyota review.

The letter from Waxman and Stupak came as Toyota openly challenged a key committee witness, who testified on February 23 that he found a possible throttle flaw during his own testing of the electronic circuitry in a Toyota Avalon.

David Gilbert, a professor of auto technology at Southern Illinois University Carbondale, has been retained by a safety advocate working with trial lawyers. Exponent said Gilbert's results were not representative of real world conditions because they could be achieved only in a laboratory setting.

Toyota has offered to demonstrate the results of our further research would welcome committee representatives to observe those demonstrations, the company said in its statement.

Waxman and Stupak also requested details about Toyota's plans for providing brake override software for countering unintended acceleration in new and some existing vehicles, as well as efforts toward making information retrievable from its vehicle data recorders, or black boxes.

(Reporting by John Crawley and Karey Wutkowski; Editing by Gerald E. McCormick, Matthew Lewis and Richard Chang)