Electric car maker Tesla had recalled more than two million vehicles across all models in the United States last December due to a risk linked to its autopilot software.

The National Highway Traffic Safety Administration (NHTSA) said it had identified in a previous investigation "at least 13 crashes involving one or more fatalities and many more involving serious injuries in which foreseeable driver misuse of the system played an apparent role".

The NHTSA said in a filing that Tesla had deployed "non-remedy updates" to address issues raised in that probe.

It said it would now investigate "why these updates were not a part of the recall or otherwise determined to remedy a defect that poses an unreasonable safety risk".

If autopilot is used incorrectly or if the driver fails to recognize that the function is activated, the risk of an accident could be higher, NHTSA has said.

US-based Tesla has been hit with several lawsuits stemming from car accidents, and its driver-assistance technology has provoked regulatory probes.

In 2021, NHTSA opened an investigation into 11 incidents involving stationary emergency vehicles and Tesla vehicles using the assisted driving feature.

Tesla has stood by the safety of its cars and autopilot features, which it has warned do not free drivers from paying attention.

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