The National Highway Traffic Safety Administration says it has opened an investigation into battery fires in two Tesla Motors Model S sedans.

The fires — three reports in six weeks — have sparked concern about the safety of the electric cars. The New York Times reports:

"Tesla also said late on Monday that it would implement certain measures aimed at creating more ground clearance in the cars and would extend its warranty policy to cover vehicles damaged by fire."The first Model S incident occurred Oct. 1, when the car struck a metal object on a highway in Kent, Wash., outside of Seattle. After a delay of more than two weeks because of a partial government shutdown, the agency decided the fire was not the result of a defect in the car's design and that an investigation was not necessary."The incident in Mexico, which is outside the federal agency's scope, happened Oct. 18. A third fire, on Nov. 6 on a Smyrna, Tenn., highway near Nashville, started after the car struck a tow hitch lying in the roadway."

The Los Angeles Times reports that Tesla Chief Executive Elon Musk said the investigation was requested by the automaker to debunk fears that the cars were more prone to fires than ordinary gas vehicles.

Tesla, the Times adds, has also changed its warranty, which now covers damage due to fire.

"Either our belief in the safety of our car is correct and this is a minor cost," Musk is quoted as saying, "or we are wrong, in which case the right thing is for Tesla to bear the cost rather than the car buyer."

Bloomberg reports that NHTSA Administrator David Strickland took issue with Musk's characterization.

"I've never heard of an automaker formally requesting an investigation," said Strickland. "And I don't think this probably happened in this case."

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