The Tennessee blaze appeared to be similar to one on Oct. 1 in Kent, Wash. In that fire, a curved piece of metal punctured a shield and the battery, which is mounted under the passenger compartment in the Model S. Experts say that if an object pierces a battery cell, that can cause sparks or arcing and touch off a fire. It's still unclear if the battery was involved in the Tennessee fire.In between those instances, a Model S caught fire in Mexico after hitting a concrete wall and a tree at a high speed. Clarence Ditlow, who heads the Center for Auto Safety and is a frequent NHTSA critic, said Friday that the agency should have immediately sent investigators to Tennessee. "Two or three similar fires (depending on whether one counts Mexico) in a vehicle population of 19,000 demands an investigation," Ditlow wrote in an e-mail. "New technology requires a heightened level of scrutiny so small problems can be nipped in the bud before they become big problems." The low-slung Model S has a 6-inch clearance between the ground and the undercarriage. Other cars with gas engines sit lower, such as the Mercedes CLA Class at 3.9 inches and a Dodge Charger at 5 inches, according to the Edmunds.com auto website. But the Tesla automatically lowers itself about another inch at highway speeds, the company's website says. "It's done in the name of improving aerodynamics to reduce drag and improve efficiency," said Edmunds.com vehicle test director Dan Edmunds. ____ Associated Press Writer Bruce Schreiner contributed from Louisville, Ky.