The U.S. National Highway Traffic Safety Administration (NHTSA) has received a surge of new complaints from Tesla Inc. owners who say their vehicle doors became inoperable after battery failures, leaving some unable to enter or exit their cars.

In an Oct. 27 letter to Tesla, the regulator said it had received additional reports just days after launching its mid-September investigation into potential door-handle defects. Owners of the company’s Model Y, Tesla’s best-selling vehicle, described exterior door handles failing to function when the low-voltage battery malfunctioned — in several cases, trapping children inside.