DETROIT (AP) - 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 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 Tes la. 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.

Musk, who has stated previously that the Model S won't be recalled, said Tuesday that if NHTSA discovers something "that would result in a material improvement in occupant fire safety," Tesla will make the change on new cars, as well as existing vehicles free of charge. He said such a discovery is "unlikely."

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 said.

In his blog post, Musk wrote that Tesla has done an over-the-air software update to give the car more ground clearance at highway speeds. The change, Musk wrote, was made to cut the chances of underbody damage, not to improve safety.

"The theoretical probability of a fire injury is already vanishingly small, and the actual number to date is zero," he wrote.

Another software upd ate in January will give the driver more control of the air suspension ride height, Musk wrote. The company also added fire damage to its warranty coverage "even if it's due to driver error."

According to the U.S. Fire Administration, there are around 194,000 vehicle fires on U.S. roads each year. The vast majority - 61 percent - start in the engine area, while 15 percent start in the passenger area. Approximately 300 people die and 1,250 are injured in U.S. vehicle fires each year. Most happen in gas-powered cars, which make up the vast majority of cars on U.S. roads. Electric vehicles make up less than 1 percent of the cars sold in the U.S.

General Motors and Nissan make the top-selling battery-powered cars in the nation, the Volt and Leaf. Neither knows of any real-world blazes in those vehicles.

Palo Alto, Calif.-based Tesla's stock rose more than 400 percent earlier in the year as the Model S won accolades from Consumer Reports and other magazines. But before Tuesday's trading, it had fallen 37 percent since the first fire was reported on Oct. 2.

The Model S can go up to 265 miles (425 kilometers) on a single charge.

The first U.S. Model S fire occurred along a freeway near Seattle when a car struck a curved metal object which pierced the shield and the battery. In the second case, a Model S caught fire Nov. 6 near Smyrna, Tenn., after the driver struck a trailer hitch in the road. Another fire was reported Oct. 17 in Mexico when a Model S burned after a high-speed crash.