The U.S. auto safety regulator investigating whether certain Tesla Inc. door handles are defective received more complaints within days of initiating its probe from consumers who were unable to enter or exit their vehicles after battery failures.

The National Highway Traffic Safety Administration cited the additional complaints in an Oct. 27 letter to Tesla about the investigation the agency initiated in mid-September. Owners of the manufacturer’s best-selling Model Y described exterior door handles becoming inoperative due to issues with their vehicle’s low-voltage battery. In several cases, this locked children inside Tesla vehicles.

The letter provides a glimpse into how NHTSA is proceeding with an investigation launched days after Bloomberg News uncovered a series of incidents in whi

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