NEW YORK -- Toyota Motor Corp. said Monday its vehicles are not at risk of accelerating out of control unless the driver's side floor mat is improperly installed or is not meant for that vehicle.

The Japanese automaker said the conclusion is the result of a U.S. government investigation into unintended acceleration of some Lexus vehicles. In August, Toyota announced a recall of 3.8 million vehicles in the U.S., including both Lexus and Toyota models, urging owners of the recalled vehicles to remove their driver's side floor mats because of the risk they could jam the accelerator pedal.

That recall -- the largest U.S. recall for Toyota -- was prompted by a high-speed crash in August involving a 2009 Lexus ES350 in which four people were killed. Occupants in the vehicle made a frantic 911 call, saying the accelerator was stuck and they couldn't stop the vehicle.

Toyota said it began mailing owners of the recalled vehicles letters about the possible problem with the mats. The recall includes the Toyota Camry for the model year 2007-2010, Toyota Avalon (2005-2010), Toyota Prius (2004-2009), Toyota Tacoma (2005-2010), Toyota Tundra (2007-2010), Lexus ES350 (2007-2010) and Lexus IS250/IS350 (2006-2010).

Toyota said there is no evidence that unintended acceleration could be caused by any defects other than an improperly installed or incorrect floor mat. It said the National Highway Traffic Safety Administration has reviewed allegations of unintended acceleration in Toyota vehicles six times, clearing the automaker each time.

"The question of unintended acceleration involving Toyota and Lexus vehicles has been repeatedly and thoroughly investigated by NHTSA, without any finding of defect other than the risk from an unsecured or incompatible driver's floor mat," said Bob Daly, senior vice president for Toyota Motor Sales USA, in a statement.