The National Highway Traffic Safety Administration (NHTSA) was sued by an auto-safety firm over the federal regulator's probe into the possible case of unintended acceleration in a 2003 Toyota Prius, the New York Times said, citing legal documents filed by the firm.

Massachusetts-based Safety Research and Strategies alleges that NHTSA withheld documentation that may have proven that the occurrence of unintended acceleration of a Prius owned by a U.S. government official may have been the result of the car's electronic systems, not the floor mats or pedals that have been subject to Toyota recalls, the newspaper reported. Safety Research is seeking documents from a May 2011 visit by NHTSA investigators to the Pennsylvania home of car owner Joseph McClelland, the Times said.

NHTSA confirmed the investigation of the 2003 Prius and the findings stating that there was no proof that the car's electronic throttle system caused the acceleration, the Times reported, adding that McClelland hasn't responded to requests for an interview.

Toyota in September 2009 recalled 3.8 million vehicles – the Japanese automaker's largest ever – because of what the company and NHTSA said were floor mats that caused some of the vehicles' accelerators to get stuck in the wide-open throttle position. The 2003 Prius that's the subject of the Safety Research lawsuit wasn't part of that recall, which included 2004-2010 model year Prius vehicles as well as Camrys, Avalons and other makes ranging between the model years 2004 and 2010.