WASHINGTON - U.S. regulators opened an investigation on Tuesday to see whether recent recalls of Toyota Motor Corp vehicles were carried out in a timely manner, the Transportation Department said.

The National Highway Traffic Safety Administration (NHTSA) requested production data, consumer complaints and other documents showing how and when Toyota learned of problems affecting about 6 million vehicles in the United States.

The three recalls involve Toyota and Lexus models.

Two of them -- in 2007 and 2009 - relate to loose floor mats that can become trapped by the accelerator. The third, in January, covers gas pedals that do not spring back as designed.

The NHTSA review could lay the groundwork for transportation officials to eventually fine Toyota if they determine the manufacturer violated its legal obligations. Fines are capped at $16.4 million.

Officials are checking to see if the manufacturer has covered all models in recent recalls of Toyota and Lexus vehicles to ensure the company did not miss any problems.

Federal law requires auto manufacturers to notify NHTSA within five days of identifying a safety defect and promptly conducting a recall.

Congressional investigators are also looking into Toyota recalls as well as the handling of those matters by NHTSA over the years. Two hearings are set for next week and a third the week after.

Safety advocates have criticized NHTSA for not being aggressive enough in responding to consumer complaints alleging safety problems.

Of the three voluntary recalls in question, only the sticky pedal case involved a defect, which the automaker is fixing.

(Reporting by John Crawley, editing by Maureen Bavdek)