Another Tesla Model S involved in a traffic crash caught fire near Smyrna, Tenn., on Wednesday after an apparent strike by road debris — the third such report in six weeks. As with the previous two cases, the news sparked a reaction from investors, who sent Tesla shares plunging, and Tesla itself, which said no one was injured and that it was investigating the cause.

But this latest incident does raise a troubling comparison for Tesla's 19,000 Model S owners: Even though it has fewer electric cars on the road than its competitors, none have reported similar fires after crashes. And while liquid-fueled vehicles suffer about 170,000 such fires every year, federal data show they take place in only 0.1 percent of all crashes.

"To have one instance of fire from road debris is a fluke," said Clarence Ditlow, director of the Washington-based Center for Auto Safety. "To have two road debris fires in a vehicle population that small is highly unusual."

On Tuesday, Tesla said it had delivered more than 19,000 Model S sedans to date, with demand running well ahead of its ability to build cars. Tesla spokeswoman Liz Jarvis-Shean said the automaker had been in contact with the Tennessee driver of the wrecked Model S "who was not injured and believes the car saved his life.

"Our team is on its way to Tennessee to learn more about what happened in the accident. We will provide more information when we’re able to do so," she said.

The photos posted to Instagram and in a Tesla owners forum show the car's front end heavily damaged, but no other vehicles nearby. The first fire of a Tesla Model S in Washington in September was due to the vehicle striking a piece of metal road debris that punctured its battery pack. The second, in Mexico last month, happened after a speeding driver lost control and struck a brick wall. U.S. auto safety officials declined to open a formal probe into the Seattle-area fire, saying the data showed no signs of any defect in the Model S.

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While the advent of mass-produced electric vehicles raised concerns over the fire safety of lithium-ion batteries, only the Tesla Model S seems to have been plagued by such reports so far. Nissan has sold nearly 70,000 Leaf electric vehicles worldwide; Nissan spokesman Brian Brockman said to date "there have been no fires involving the Nissan LEAF, either through extensive and extreme testing or in the real world." Last year, a Nissan Leaf was caught in a Colorado wildfire which engulfed the vehicle — but its lithium-ion battery pack did not burn.

The only other electrified car with a comparable base of sales would be General Motors' Chevy Volt, which uses a gas-powered engine to recharge a large lithium-ion battery pack. So far, GM has sold roughly 50,000 Volts worldwide, most of them in the United States. While two Volts did catch fire in testing in 2010 by the National Highway Traffic Safety Administration, GM issued a recall to fix a cooling issue. Since then, "to GM's knowledge the Volt has not experienced a fire on the road," said spokeswoman Michelle Malcho.

No other automaker selling EVs today has reported a fire connected to one in a crash. The $100,000 Fisker Karma plug-in hybrid suffered several cases of fires when at rest, including 16 burned when they were flooded in Hurricane Sandy; the company has since halted production.

The Volt, Leaf and Model S all use lithium-ion batteries wrapped in protective steel shells designed to prevent fires in case of a crash, although the Model S carries as much as three times the amount of stored electrical energy as the Volt and up to five times more than the Leaf. Automakers and battery providers typically put those batteries through a series of destructive tests to ensure fire resistance; GM even shot at its Volt battery packs to ensure random gunfire wouldn't trigger flames, and Tesla coats its battery cells in fire-retardant chemicals.

But the design of the Tesla Model S differs from the Volt, Leaf and all other EVs for sale today in one key respect. Tesla placed the Model S battery pack along the vehicle's underside, where it could be easily accessed and even swapped quickly by an automated system Tesla has said it would offer in the future for five-minute recharging. Most other EVs, typically based on the chassis of a gas-powered model, place their battery packs toward the rear of the vehicle where the fuel tank would normally go; in the Volt, the pack was designed with a T-shape running in the center of the car. Such designs keep the packs away from the front of the vehicles — where roughly 70 percent of crashes strike — and leave a smaller area for debris to hit.

NHTSA said Thursday it would get in touch with crash investigators in Tennesse to see whether it needs to take any additional steps. Ditlow says the data so far suggest NHTSA should open a formal probe — but that the Tesla fires don't pose the danger usually associated with a vehicle accident.

"The good news is it's not a crash where you're knocked senseless or can't get out," said Ditlow. "The potential for death or injury is relatively low in these vehicles. The fires are of a nature where you have enough warning to get off the road.

"The biggest concern to Tesla owners would be that they need a new vehicle."