Toyota said in a statement that it is cooperating with NHTSA in the investigation. Customers with concerns should call Toyota at (800) 331-4331.
Most of the six fires were minor with damage limited to the doors, but a Camry was destroyed in one case, according to complaints filed with NHTSA. Several owners reported that they were afraid to drive their vehicles because of the threat of fires.
In one case, on Nov. 19, 2011, a Camry owner reported seeing flames coming from the door right after starting the car. Firefighters were called, but the car was reported destroyed, the complaint said.
A RAV4 owner reported that in August of last year, the master power window switch caught fire, burning a hole the size of a dime. The owner had been having trouble with the power window since July of 2009.
"I was so afraid to drive the fire trap I traded it in for a new car," the complaint said. "The RAV4 was paid off. Now I have a car loan at 19.9 percent."
Names and locations are not listed on the complaints.
Toyota's reputation has taken a hit over the past three years due to a string of huge recalls that have ballooned to more than 14 million vehicles worldwide. Millions were recalled for acceleration problems, and Toyota replaced floor mats that can trap gas pedals and pedal assemblies that can stick and cause cars to take off by surprise. But after an exhaustive probe, U.S. safety regulators, aided by NASA engineers, found nothing wrong with Toyota's electronic throttle controls.
Still, the latest investigation is troubling for the automaker because the Camry is consistently is the top-selling car in the U.S., and the RAV4 also is a big seller. The probe also includes the Solara, which is a coupe version of the Camry.
Toyota sold 473,108 Camrys and 172,752 RAV4s in 2007, according to Autodata Corp. Some 2007 models may have been sold in 2006.
In December, the 2012 Camry received a five-star safety rating from the National Highway Traffic Safety Administration, the agency's top rating.