Toyota Motor Corp has not adequately supported its contention that it has rigorously evaluated electronic throttle systems for possible problems, lawmakers said on Friday.

Henry Waxman and Bart Stupak, the chairman of the House of Representatives Energy and Commerce Committee and its investigative subcommittee, respectively, said in a letter to Toyota's U.S. sales chief, Jim Lentz, that documents turned over to lawmakers so far contain only cursory information.

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 turned over to the committee in recent weeks 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 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 regulators and safety advocates have questioned whether there are glitches in throttle software as well.

The National Highway Traffic Safety Administration (NHTSA) is reviewing the matter, but previous investigations by the agency found no problems with Toyota throttles.

Toyota executives told congressional hearings in recent weeks that throttle systems have been exhaustively tested without showing any problems.

Waxman was critical of an interim report from a Toyota consultant that was provided to the committee in February, asserting no electronic throttle problems. Toyota has told lawmakers at hearings that the independent review is in its early stages.

The committee asked Toyota to identify officials with personal knowledge of electronic throttle testing as well as quarterly reports detailing any allegations of unintended acceleration.

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 about making information retrievable from its vehicle data recorders, or black boxes.

(Reporting by John Crawley and Karey Wutkowski, editing by Gerald E. McCormick)