DETROIT (AP) — U.S. highway safety investigators want to know why Tesla didn’t file recall paperwork when it updated Autopilot software so it would do a better job spotting parked emergency vehicles.

In a letter sent to Tesla, the National Highway Traffic Safety Administration told the electric car maker that it has to do a recall if an over-the-internet update mitigates a safety defect.

The actions show an escalating confrontation between Tesla and the agency as it moves to regulate partially automated driving systems.

In August the agency opened an investigation into Autopilot after getting multiple reports of Teslas crashing into emergency vehicles parked on highways with warning lights flashing. 

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