Federal auto safety regulators opened an investigation Tuesday into possible defects in Tesla doors that have reportedly left parents with children trapped in the back seat and forced to break windows to get them out.

The National Highway Traffic Safety Administration said the preliminary probe is focused on 2021 Tesla Model Ys after receiving nine reports of electronic door handles not working possibly due to low battery voltage.

The company run by billionaire Elon Musk has installed manual door releases inside the vehicles but NHTSA noted that a child may not be able to reach or know how to operate the releases. In four cases, the parents had to break the windows to get inside.

The investigation into Tesla’s most popular model comes after numerous reported incidents in recent years of other problems with opening Tesla doors, sometime trapping drivers in burning vehicles after accidents and a loss of power.

In April, a college basketball recruit said he was “fighting time” trying to get out of his Tesla Cybertruck that had caught fire after he had crashed into a tree and was unable to get the doors open. The University of Southern California player, Alijah Arenas, who was induced into a temporary coma after the accident, said he stayed alive by dousing himself with a water bottle as smoke filled the vehicle.

Last year, relatives of the driver of a Tesla Model 3 who was trapped in his car and burned beyond recognition sued Musk’s company for negligence and fraud for failing to fix what they called a design flaw in the doors despite what they said were 200 fires involving its cars. The case is pending 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.