Washington DC - The advanced version of Tesla Inc.'s partially automated driving assistance system, which can also be used in city traffic, is being scrutinized by the US National Highway Traffic Safety Administration (NHTSA).

The NHTSA said on Thursday it is probing the failure of Tesla's full self-driving (FSD) equipped electric vehicles' system engineering controls to react appropriately to reduced roadway visibility conditions.

The auto safety watchdog is probing about 2.4 million vehicles that include the 2016-2024 Model S and X, 2017-2024 Model 3, 2020-2024 Model Y, and 2023-2024 Cybertruck equipped with FSD.

The probe was initiated after four reports of FSD collisions in reduced roadway visibility conditions, such as sun glare, fog, or airborne dust. FSD is optionally available on these vehicles.

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In one of the collisions, the Tesla vehicle fatally struck a pedestrian, and another crash in these conditions involved a reported injury, the NHTSA said.

The NHTSA noted that it will assess the ability of FSD's engineering controls to detect and respond appropriately to reduced roadway visibility conditions.

It will also assess whether any other similar FSD crashes have occurred in reduced roadway visibility conditions and, if so, the contributing circumstances for those crashes.

The watchdog will also probe if the performance of FSD in reduced roadway visibility conditions was affected due to any updates or modifications from Tesla to the FSD system.

Last week, Tesla founder and chief executive Elon Musk unveiled the Cybercab robotaxi self-driving concept car concept vehicle at Warner Bros Studios at Burbank in California.

It showcased a silver two-seater, without any steering wheels or pedals.

Tesla said it is hoping to produce the Cybercab before 2027.

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Musk noted that the car would be capable of driving itself autonomously when it launches, and that consumers would be able to buy one for a price tag under $30,000.

Musk further said that he expects Tesla to have unsupervised FSD up and running in Texas and California next year in the company's Model 3 and Model Y electric vehicles.

FSD, Tesla's premium driver assistance system, is currently available in a supervised version for Tesla electric vehicles, and requires a human driver at the wheel.