Representatives Henry Waxman, the chairman of the Committee on Energy and Commerce, and Bart Stupak, who heads the panel's oversight subcommittee, said documents that Toyota had turned over so far "did not provide convincing substantiation" exonerating the electronic system. The lawmakers asked for more information by March 12.

"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 unintended acceleration," Mr. Waxman, Democrat of California, and Mr. Stupak, Democrat of Michigan, wrote in a letter to the president of Toyota Motor Sales U.S.A,, James E. Lentz III.

Toyota said in its response that executives "have received the letter and will, of course, cooperate."

Mr. Lentz, in testifying before the committee Feb. 23, said the repairs that Toyota dealerships were making on accelerator pedals might "not totally" resolve problems with unintended acceleration. Toyota later attempted to clarify Mr. Lentz's statement to mean that cruise control, "pedal misapplication" and other factors not specific to Toyota still could cause unintended acceleration.

Toyota has insisted on several occasions — to customers as well as to Congress — that it has rigorously tested and found no flaws in the electronic throttle control systems, which substitute for a mechanical link between the accelerator pedal and the engine in Toyotas and many other vehicles.

But the possibility that electronics are to blame has continued to dog Toyota, particularly as federal safety regulators have begun to receive reports of vehicles experiencing problems even after the recall repairs. As of Thursday, the National Highway Traffic Safety Administration had received at least 60 complaints of unintended acceleration in Toyotas that supposedly had been fixed, and officials said they were trying to contact those drivers for more information.

The safety agency warned Toyota that it could order a new repair process if the current one was not effective. Toyota dealers have repaired more than a million vehicles in the last month, a process that, depending on the particular model, could involve adding a metal spacer into the pedal, shortening the pedal or removing some padding under the floor to prevent the pedal from becoming trapped under the mat. It is also adding a brake-override system that allows the brakes to supersede the accelerator if both pedals are depressed simultaneously.

Late Thursday, Toyota issued a statement noting that the new reports involved a "tiny fraction" of the repaired vehicles but that it was looking into them. It asserted that its vehicles "are among the safest on the road today" after being repaired.

"Although most of these reports have yet to be verified, Toyota has been and remains committed to investigating all reported incidents of sudden acceleration in its vehicles quickly," the statement said. "Toyota wants to hear directly from its customers about any problems they are experiencing with their vehicles."

Mr. Waxman and Mr. Stupak asked Toyota to identify employees "with personal knowledge of Toyota's efforts to test its vehicles for electronic defects that could cause sudden unintended acceleration" so the panel could interview them next week. They also requested that Toyota provide quarterly reports about allegations of unintended acceleration it has received, with the first due March 15.