The most recent fire, the third in five weeks, occurred Wednesday in Tennessee, CNN reported Friday. The Tennessee Highway Patrol said the fire was caused by the impact of a metal object, a tow hitch.
"The tow hitch hit the undercarriage of the vehicle causing an electrical fire," a Highway Patrol statement said. "The driver was able to pull the vehicle over onto the left side emergency lane and come to final rest."
A fire reported in Washington state in early October appeared to have a similar cause. The driver also got out without injury.
A Tesla S caught fire in Mexico after hitting a wall.
The National Highway Traffic Safety Administration decided against reviewing the first fire and has no jurisdiction in Mexico. But the agency plans to send investigators to Tennessee to examine the car there, The Detroit News reported.
Clarence Ditlow of the Center for Auto Safety urged NHTSA to conduct a formal investigation. He told the News the Tesla, which has its battery pack placed under the floor, could need more shielding.
Sean Kane, founder and president of Safety Research & Strategies, agreed the fires could indicate a problem with the Tesla. Kane's company works with plaintiffs' lawyers in car safety litigation.
"These are all issues that have got to get resolved," Kane said. "At the end, we do see fires in gasoline-powered vehicles but not a lot from undercarriage impacts. That's a pretty rare event. It does raise questions. Are we looking at something that is a bigger problem?"
Tesla says about 19,000 Model S vehicles are now on the road. In August, NHTSA gave the Model S its highest possible score on crash tests.