A Tesla Model S erupted 'like a flamethrower.' It renewed old safety concerns about the trailblazing sedans.

The sound was like "if you were to drop an axle of a normal car" on the ground, Ahmad, 41, said. Only the car was intact, he recalled. Suddenly, as he stood on the side of the road, the car ignited in flames, concentrated around the front passenger-side wheel. "This was shooting out like a flamethrower," recalled Ahmad, who works in strategy and business development for a health-care system.

The combustion of Ahmad's car is one of a growing number of fire incidents involving older Tesla Model S and X vehicles that experts say are related to the battery, raising questions about the safety and durability of electric vehicles as they age. The National Highway Traffic Safety Administration (NHTSA) is evaluating the fire of Ahmad's vehicle in Frisco, Tex., and has contacted Tesla over the matter, NHTSA spokesman Sean Rushton said this month. The agency opened an investigation last year into alleged battery defects that could cause fires in older Tesla sedans and SUVs.

Tesla did not respond to requests for comment sent to multiple representatives.

A lawsuit and defect petition that spurred the NHTSA probe allege Tesla manipulated its battery software in older model cars to reduce the risk of fire, lowering the range and lengthening charging times as it sought to address an undisclosed defect. The attorney filing suit on behalf of Tesla owners last year cited an "alarming number of car fires" that appeared to be spontaneous. Since the agency agreed to look into the issue last year, little more has been disclosed about the status of the probe.

Tesla has argued its cars are 10 times less likely to catch fire than gasoline vehicles, citing data from the National Fire Protection Association and U.S. Federal Highway Administration on the number of incidents by mileage traveled for its fleet of electric cars vs. other vehicles. Tesla said in 2018 that its vehicles had five fires per billion miles traveled, vs. 55 fires per billion miles traveled in the United States.

Other electric vehicle models have faced federal scrutiny and voluntary recalls over fire risks. Last month, NHTSA announced General Motors was recalling more than 50,000 Chevrolet Bolt electric cars in the United States over the potential for fire in its high-voltage battery pack, after the agency confirmed there were five known fires involving the vehicle, resulting in two injuries. NHTSA advised owners to park their cars outside until the problem is repaired.

General Motors spokesman Daniel Flores said dealers were updating the cars' battery software to limit their charge capacity to 90 percent while the company addressed the issue. The batteries, he said, "may pose a risk of fire when charged to full, or very close to full, capacity," and the company is "working around-the-clock to identify the root cause."

Audi recalled its e-tron SUV last year shortly after its U.S. launch following the discovery of a potential fire risk, which the company said was a wiring harness issue. Audi spokesman Mark Dahncke said that Audi recorded no fires globally and that the recall was done out of an abundance of caution.

And federal regulators investigated General Motors for battery fire risks in its plug-in hybrid Chevrolet Volt in 2011, a problem GM agreed to address.

There were 189,500 highway vehicle fires in the United States in 2019, according to the National Fire Protection Association, encompassing passenger and other types of road-going vehicles. Experts say electric cars catch fire at a similar rate to gas cars, if not less often. But the duration and intensity of the fires, fueled by chemicals and the extreme heat buildup in lithium-ion battery systems, can make the fires in electric cars harder to put out.

"Battery fires can take up to 24 hours to extinguish," Tesla says in an emergency response guide for the Model S on its website. "Consider allowing the battery to burn while protecting exposures."

As a report prepared for NHTSA suggests, electric vehicle fires can result from a chain reaction of events where, for example, a defect causes overheating in a single cell. Through that vector, the heat can ignite highly flammable materials surrounding the source and spread to the rest of the battery, eventually spiraling out of control as temperature and pressure rise unabated, a process known as "thermal runaway." But the issue may not be inherent to batteries, but rather the fact that the current crop of electric vehicles are relatively new to market and uniform safety standards have yet to be adopted, research has said.

An October 2017 Battelle report prepared for NHTSA on the safety of lithium-ion batteries for electric and plug-in hybrid cars "suggests that the technology and industry has not matured sufficiently to have established comprehensive safety codes and standards that mitigate risks."

Tesla has come under particular scrutiny over concerns its computerized cars made emergency responses and investigations more difficult, with features such as retracting door handles that proved an impediment to first responders, for example, and proprietary systems with critical incident information that have required Tesla's cooperation to decode.

The NHTSA defect petition that led to the probe cited alleged "high-voltage battery fires that are not related to collision or impact damage to the battery pack." It focused on Model S and X vehicles from model years 2012 through 2019 and homed in on their battery management systems, including thermal management and charging control, NHTSA said.

One of the most gruesome incidents involving the Model S was the case of driver Omar Awan, who was trapped in a burning car in South Florida in 2019 after the car's electronic door handles failed to retract following a fiery crash, his family said. The man's family blamed that design feature in a wrongful-death lawsuit, saying his death was caused by the design features rather than the crash itself.

The battery reignited at least three times in the impound lot, according to the South Florida Sun-Sentinel.

Another fatal wreck in South Florida, in 2018, led the family of a teenage victim to sue Tesla, alleging the battery pack was defective. The firm representing that family alleged there were at least a dozen cases of Model S batteries igniting after a collision or while parked.

In 2019, Tesla said it sent investigators to the site of a Model S explosion in a Shanghai car park after video showed smoke billowing from the parked car before a fiery blast.

And a Tesla Model S burst into flames while sitting in traffic on a Los Angeles street in 2018, with fire shooting similarly from the wheel well. Tesla called it an "extraordinarily unusual occurrence."

The Frisco incident bore resemblances to many of the previous cases, although Ahmad had been driving the Tesla Model S 85D shortly before it ignited. Ahmad said his battery was at around a 60 percent charge, he said, and he was cruising lightly on the way home from Home Depot.

Firefighters showed up within minutes after Robert Watson, 41, of Frisco witnessed black smoke and called 911. One of Watson's sons started recording the fire, which was growing in intensity.

"It looked like the back of a jet engine with the afterburner on coming out of that front passenger wheel," recalled Watson, who works for a technology company. The firefighters had the blaze under control in about 10 minutes, witnesses recalled.

But there was another issue. As he stood on the side of the road, Ahmad said, a firefighter asked how to get inside the cabin as they worked to douse the flames.

Ahmad thought to try the key fob but knew it might be futile. The Model S uses retractable door handles that are electronically controlled, popping out when they detect a nearby fob.

The firefighter "looked at me and he said, 'You're lucky you got out when you did, because you could have gotten stuck in there,'" Ahmad said. It raised similar concerns to the Florida crash involving Awan.

Keith Gall, battalion chief of administrative services for the Frisco Fire Department, told The Washington Post the fire involving Ahmad's Tesla was deemed "unintentional," though he did not elaborate. The car was destroyed, Ahmad said, and is sitting in an insurance lot. Now he is awaiting answers on the potential cause, though he said Tesla had not initially proved eager to investigate the fire.

"I am assuming the battery exploded and caused the fire, but would like to request Tesla please look into this," he wrote Nov. 25 to a Tesla service center representative, who later responded they were glad he was safe and would look into the matter.

Since then, Tesla has repeatedly cited insurance hurdles. Once Ahmad files a claim, the parties will agree to jointly inspect the car, one Tesla representative told him.

"Until this is done, there is no time frame," the Tesla rep told him in an email shared with The Post. A month after Ahmad's email to Tesla, the company and his insurance agency agreed to jointly inspect the car, Ahmad said.

Ahmad said he wants to determine the root cause of the problem so this doesn't happen to someone else.

Ahmad fears for others who have the same model vehicle and even for his parents, whom he convinced to get a Tesla Model X SUV.

"I don't want anybody else to experience something this scary," he said.