The National Highway Traffic Safety Administration (NHTSA) is investigating Tesla's semi-autonomous self-driving software, over concerns that it lacks the ability to detect road hazards in low-visibility environments. 

Roughly 2.4 million Tesla vehicles currently have the automaker's "full self-driving" (FSD) software. Despite what the name might imply, FSD still requires a human driver to actively supervise the road, and is not fully autonomous. Rather, it guides vehicles to ramps on and off highways, automatically changes lanes when a driver signals, and slows vehicles at stop signs and traffic lights, among other functions.

According to a report from the NHTSA, at least four collisions linked to FSD issues have been reported to the agency, including one instance where a Tesla vehicle hit and killed a pedestrian. In each instance, the vehicles had their FSD systems turned on during limited visibility caused by factors such as fog and glare from the sun. 

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The NHTSA's investigation will look into the FSD system's ability to "detect and respond appropriately to reduced roadway visibility conditions," and whether there might be other similar incidents where a Tesla using FSD crashed in low visibility. If the agency finds that the system poses enough of a safety risk, it could potentially order a full recall of all vehicles that use the software, which would include all of Tesla's passenger models dating back to 2016.