Federal investigators have started a probe into the fire risk for a Tesla Model S.

The announcement comes after three fires in the last six weeks involving the electric car, which has gotten top safety ratings in crash tests performed by the National Highway Traffic Safety Administration.

Just last month, the agency said it was not planning to conduct a probe because "after reviewing all available data," it had not found evidence showing the fires were the result of a problem with the car.

In congressional testimony Tuesday, NHTSA Administrator David Strickland said the agency decided to open the investigation because of similarities between the two fires that took place in the United States.

"Two being a trend, we clearly saw some issues," he said during a hearing on an unrelated topic.

Tesla Motors CEO Elon Musk said Tuesday that the company's vice president of regulatory affairs had requested the probe on Friday to combat any doubts or fears about the fire risk.

"Given that the incidence of fires in the Model S is far lower than combustion cars and that there have been no resulting injuries, this did not at first seem like a good use of NHTSA's time," the company said on its blog. "However ... if a false perception about the safety of electric cars is allowed to linger, it will delay the advent of sustainable transport."

But Strickland testified that NHTSA independently decided to start the probe without a request from Tesla.

Related: Are Tesla shares a bubble or a bargain

Tesla (TSLA) shares have lost about a third of their value since the first fire was reported. The stock, which fell 10% on Monday, was up 4% in midday Tuesday trading. Shares are still up 275% this year.

Tesla also said it has updated the car's software to change the car's air suspension so that it will have greater ground clearance at highway speeds.

The first and third fires occurred when the cars struck debris in the road. The company said another software update expected in January will give the driver direct control of the air suspension ride height.

CEO Elon Musk has defended the safety record of the Model S, pointing out that no one has been hurt or killed in any accident, and that the fires occur about one-fourth as often as in the case of a traditional gasoline-powered car.

There are about 19,000 Model S vehicles on the road, meaning that about one in 6,300 has had a fire.

By comparison, there are about 250 million cars and trucks, virtually all of them gasoline powered.

According to the latest figures from the National Fire Protection Association, there were about 172,500 vehicle fires in the United States last year, or about one for every 1,450 vehicles on the road. Those fires were associated with 300 deaths and 800 injuries. There was a fire involving gasoline powered cars about every three minutes on average last year.

Related: After fire, Tesla driver says he'd buy another

Last week, Tesla CEO Elon Musk said "there's no reason" for a recall. "If you read the headlines, it sounds like Teslas have a greater propensity to catch fires than other cars. In reality, nothing could be further from the truth."

On Tuesday, he tweeted, "Why does a Tesla fire with no injury get more media headlines than 100,000 gas car fires that kill hundreds of people per year?"

Musk said the fires were caused by the severity of the accidents they were involved in, not a problem with the car. He said if a recall could fix any problem, Tesla would be quick to order one.

There have been two recalls of Tesla models in the past. The problems that were addressed did not cause any accidents or injuries.