DETROIT  Lawmakers and the Transportation Department stepped up pressure Tuesday on Toyota, seeking proof that problems that could cause its cars to speed up unexpectedly were limited to floor mats and sticking pedals.

In a letter to James E. Lentz III, the president of Toyota Motor Sales U.S.A., Representatives Henry A. Waxman and Bart Stupak asked the automaker to provide documents showing that the computer systems on its cars were not at fault  something Toyota has vigorously denied.

Mr. Waxman is the chairman and Mr. Stupak a subcommittee chairman of the House Energy and Commerce Committee.

Meanwhile, the Transportation Department said it was considering a civil penalty against Toyota over the handling of the recalls, a person with direct knowledge of the discussions said.