The CEO of electric car company Tesla said Friday that a battery in a Model S that caught fire this week was apparently impaled by a metal object.

Elon Musk gave more detail in a blog post about the fire that became an Internet sensation and unsettled Tesla investors. He also defended the car’s battery technology.

Musk wrote in a blog post Friday that fires are more common in conventional gas-powered vehicles.

“For consumers concerned about fire risk, there should be absolutely zero doubt that it is safer to power a car with a battery than a large tank of highly flammable liquid,” Musk wrote.

The CEO said a curved metal object on the road was apparently to blame for the fire Tuesday. He says the large object’s shape led to a powerful hit on the underside of the vehicle, punching a 3-inch hole through an armor plate that protects the battery under the passenger compartment.

The car properly contained the blaze in one section of the battery, the company said. The driver was able to exit the highway in the Seattle suburb of Kent and get out of the vehicle before flames engulfed the front of the sedan.

Of the estimated 194,000 vehicle fires in the U.S. each year, the vast majority are in cars and trucks with gasoline or diesel engines. Electric vehicles make up less than 1 percent of the cars sold in the U.S.

Firefighters struggled to extinguish the Tesla fire, finding that the flames reignited. After dismantling the front end of the vehicle and puncturing holes in the battery pack, responders used a circular saw to cut an access hole in the front section to apply water to the battery, according to documents. Only then was the fire extinguished.

Musk said in his blog post that a “road crew” at the scene of the fire identified the curved metal piece, which fell off a tractor-trailer, as the apparent culprit. However, state and local officials have not been able to say what debris, if any, was found in the area.

Shares of Palo Alto, Calif.-based Tesla Motors Inc. fell sharply Wednesday and Thursday after a video of the car fire circulated on the Internet. The stock recovered somewhat Friday, rising $7.67, or 4.4 percent, to $180.98. The shares still finished the week with a loss of $9.92, or 5.2 percent, but they’re still up nearly 400 percent this year.