The National Highway Traffic Safety Administration has opened an investigation into claims that Tesla’s door handles become inoperable in certain situations on Model Y SUVs.

The safety agency’s Office of Defects Investigation (ODI) revealed the probe on Tuesday, after having received nine reports from owners who were unable to get into their cars. ODI writes that the most common scenario involves parents who exit their car and cannot open the rear doors to remove their children.

In four of those cases, according to the ODI report, owners had to break a window “to regain entry into the vehicle.”

News of the probe comes a few days after Bloomberg published an investigation into a series of incidents where Tesla owners (and passengers) became stuck inside their vehicles following a crash.

According to a preliminary review by ODI, Tesla’s handles can become inoperable if the electronic door locks aren’t getting enough voltage from a vehicle’s battery system. But ODI also points out in its preliminary report that none of the owners who related this problem to the agency mentioned seeing low-voltage battery warnings.

Tesla’s vehicles do have manual door releases, although they are only on the inside of the car. The door releases can be hard for children to access, and some owners aren’t even aware they exist, according to ODI.

It’s possible to restore power to the electronic door locks, according to Tesla’s owners manuals, but it’s a multi-step process that requires an external power source.

Tesla did not respond to a request for comment.