By TOM KRISHERDETROIT (AP) a¿¿ The U.S. government's auto safety watchdog is investigating whether Tesla's Model S electric car is vulnerable to fires because roadway debris can pierce the car's underbody and battery. The National Highway Traffic Safety Administration, which announced the probe early Tuesday, is looking into two incidents in which Model S drivers struck metal objects on highways. The objects penetrated the bottom of the car, punctured the battery and caused fires. Both drivers were warned of a problem by the car and escaped safely. Tesla CEO Elon Musk said in a blog post that he requested the NHTSA investigation. He says accident data show that the Model S is far safer than gasoline-powered cars, but the probe is needed to dispel questions the public may have about the safety of electric vehicles as a result of the fires. But NHTSA Administrator David Strickland told reporters in Washington Tuesday that he isn't aware of any request from Tesla. However, the agency said Tesla is cooperating in the investigation. News of the probe didn't hurt Tesla's stock price. It gained $4.51, or 3.7 percent, to close at $126.09, and at one point rose to $129. The probe affects more than 13,000 cars from the 2013 model year that were sold in the U.S. Tesla has sold about 19,000 of the cars worldwide. They start at $70,000 but often run more than $100,000. Tesla's batteries are mounted beneath the passenger compartment and protected by a quarter-inch-thick metal shield. Experts say that if the batteries are damaged, that can cause arcing and sparks and touch off a fire. NHTSA, in documents posted on its website, said it opened the preliminary evaluation "to examine the potential risks associated with undercarriage strikes" on the Tesla cars. The investigation could lead to a recall, but a decision likely is months away.