Federal auto safety regulators initiated an investigation Tuesday into potential issues with Tesla doors that reportedly have resulted in children being trapped in the back seat, compelling parents to break windows to free them.

The National Highway Traffic Safety Administration announced the initial probe targets 2021 Tesla Model Ys after receiving nine reports of electronic door handles failing, possibly due to low battery voltage.

The business, led by billionaire Elon Musk, has equipped vehicles with manual door releases. However, NHTSA pointed out that a child might not be able to access or know how to use these releases. In four instances, parents were forced to break windows to gain entry.

The inquiry into Tesla’s most popular model comes after numerous reported incidents in recent years concerning issues with opening Tesla doors, sometimes trapping drivers in burning vehicles after accidents and power failures.

In April, a college basketball recruit recounted the desperate struggle to escape his Tesla Cybertruck, which caught fire after crashing into a tree, rendering the doors inoperable. The University of Southern California player, Alijah Arenas, who was induced into a temporary coma following the accident, survived by using a water bottle to protect himself as smoke filled the vehicle.

Last year, relatives of a Tesla Model 3 driver who perished being trapped and burned beyond recognition filed a lawsuit against Musk’s company for negligence and fraud, accusing it of failing to address what they termed a door design flaw despite allegedly 200 fires involving its vehicles. The case is underway in Los Angeles Superior Court.

NHTSA said the investigation is only focusing on the operability of the electronic door locks from outside of the vehicle, not inside, as that’s the only instance in which there is no manual way to open the door. But it also said it will continue to monitor reports of people stuck on the inside – what it calls “entrapment” – and will take further action as needed.

The current agency investigation covers approximately 174,300 of the midsize SUVs.

The agency said the incidents appear to occur when the electronic door locks receive insufficient voltage from the vehicle. It said that available repair invoices indicate that batteries were replaced after such incidents took place. Of those who reported incidents, none saw a low voltage battery warning before the exterior door handles became inoperative.

NHTSA said that its preliminary evaluation will look at the scope and severity of the condition, including the risks that come from the conditions that have been reported to them.

The agency’s investigation will also assess the approach used by Tesla to supply power to the door locks and the reliability of the applicable power supplies.

Tesla is also under investigation by NHTSA for a “summon” technology that allows drivers to tell their cars to drive to their location to pick them up, a feature that has reportedly led to some fender benders in parking lots. A probe into driver-assistance features in 2.4 million Teslas was opened last year after several crashes in fog and other low-visibility conditions, including one in which a pedestrian was killed. Another investigation was launched by NHTSA in August looking into why Tesla apparently has not been reporting crashes promptly to the agency as required by its rules.

Musk is under pressure to show that the latest advances in its driver-assistance features have not only fixed such reported glitches but have made them so good drivers don’t even need to look out the window anymore. He recently promised to put hundreds of thousands of such self-driving Tesla cars and Tesla robotaxis on roads by the end of the next year.

That effort has become more urgent as a plunge in Tesla sales that began earlier this year shows little sign of abating. Musk has angered the traditional environmentally conscious Tesla customer base by supporting U.S. President Donald Trump and far-right politicians in Europe and their policies. On Saturday, he told protestors at an anti-immigration rally in London that “violence is coming to you” and “you either fight back or you die.”

Tesla stock rose more than 2% to $419.25 in afternoon trading Tuesday following a jump the day before on news Musk had bought $1 billion worth of shares.