WASHINGTON — Tesla Motors’ third fire in five weeks involving a Model S suggests U.S. regulators need to examine the luxury electric car, a safety advocate said after a Tennessee accident.

Shares of the Palo Alto, Calif.-based carmaker led by Elon Musk slid as much as 9 percent to $137.62, after reports of the newest fire. That decline follows a 15 percent drop Wednesday after the carmaker’s third-quarter results and fourth- quarter outlook disappointed investors. The two-day drop is the biggest intraday decline since Dec. 27, 2010.

The National Highway Traffic Safety Administration “absolutely has to investigate” Wednesday’s Tennessee incident, Clarence Ditlow, executive director of the Center for Auto Safety, based in Washington, said in a telephone interview.

A Model S driver struck a tow hitch in the middle of a lane on Interstate 24 near Murfreesboro, Tenn., damaging the car’s undercarriage and causing the fire, Dalya Qualls, a Tennessee Highway Patrol spokeswoman, said in an email. The driver was able to pull the car over and was uninjured, Qualls said.

NHTSA declined to investigate the Model S fire in Washington state in October, the first such reported blaze, in which metal debris was involved.

“It appears there’s inadequate shielding on the bottom of these vehicles,” Ditlow said. “Road debris is a known hazard to the undercarriage of vehicles.”

The U.S. agency “will contact the local authorities who are looking into the incident to determine if there are vehicle safety implications that merit agency action,” Karen Aldana, a spokeswoman for NHTSA, said in an email.

Liz Jarvis-Shean, a Tesla spokeswoman, didn’t immediately respond to an email question about the need to strengthen the aluminum casing that houses the 1,000-pound lithium-ion battery that powers the Model S.

“We have been in contact with the driver, who was not injured and believes the car saved his life,” Jarvis-Shean said in an earlier email. “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.”

Tesla, the best-performing automotive stock this year, has been under scrutiny as Musk works to create the world’s biggest and most profitable seller of electric cars. Even with this week’s declines, Tesla shares have surged more than fourfold this year after reporting its first quarterly profits.

SOLUTION COULD BE SIMPLE

The Washington state Model S fire took place Oct. 1, with another in Mexico in mid-October. NHTSA said Oct. 24 it found no evidence the first Model S fire resulted from defects or violations of U.S. safety standards. NHTSA doesn’t investigate vehicle accidents that occur outside the U.S., the agency has said.

A possible fix for the Model S, priced from $70,000 to more than $100,000, “is not rocket science,” Ditlow said. “Probably the simplest task Tesla has is putting a strong steel shield on the bottom of the car.”

A U.S. investigation or recall of the Model S “could raise near-term concerns to a higher level in terms of cost, image and production disruption to Tesla,” Adam Jonas, an analyst for Morgan Stanley, said in a research report Thursday.

“We would be very surprised if Tesla has not already launched its own investigation into the cause of battery compartment intrusion to explore causes and potential remedies,” said Jonas, who rates the stock overweight. A voluntary recall “would be consistent with Tesla’s extraordinary efforts to build brand value and trust.”

Multiple reports of battery fires may harm Tesla’s image, said Karl Brauer, an industry analyst for Kelley Blue Book in Irvine, Calif.

“At some point the cause of the fire, the safety of the drivers and even the attitude of the owners – all three apparently want another car – stops mattering because you’re left with recurring headlines featuring the words ‘Tesla’ and ‘fire,’ ” Brauer said.

“For a company with a stock price based as much or more on image than financials, those recurring headlines are highly damaging,” he said.