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Tensions between Tesla and the National Highway Traffic Safety Administration are escalating. Safety officials are questioning the automaker about its decision not to issue a recall during a critical software update.

Tesla updated its Autopilot software in late September to better identify parked emergency vehicles, and the NHTSA says that safety defect should have triggered a recall, even if the update was done over the Internet.

In a letter to Eddie Gates, Tesla’s director of field quality, the agency said “any manufacturer issuing an over-the-air update that mitigates a defect that poses an unreasonable risk to motor vehicle safety is required to timely file an accompanying recall notice to NHTSA.”

Safety officials opened a formal investigation of Tesla’s Autopilot in August, following a series of collisions with parked emergency vehicles. Overall, Tesla cars struck first responder vehicles 12 times leading to injuries and vehicle damages.

Elon Musk shrugged off that investigation, announcing days later that Tesla would roll out full self-driving to all owners last month. This feature allows drivers to take their hands off the steering wheel while the car pilots itself around city streets using only its cameras to navigate a highly complex environment with pedestrians, pets, bikers, and other cars. Since it is merely an assistance tool rather than a true autonomous self-driving system, liability remains at all times with the driver in the event of an accident—rather than being transferred to Tesla.

The escalation between Tesla and the NHTSA follows controversy from earlier this year, when Consumer Reports pulled its safety rankings from some Tesla models and the Insurance Institute for Highway Safety removed its Top Safety Pick+ designation. Both decisions came after the NHTSA stopped giving the the Models 3 and Y check marks on its website for having forward collision warning, automatic emergency braking, lane departure warning, and emergency brake support.