The National Highway Traffic Safety Administration (NHTSA) dealt a potential setback to Tesla’s (TSLA) autonomous ambitions with a new investigation into the company’s full self-driving (FSD) technology.

Specifically, NHTSA said it had opened an investigation into Tesla’s FSD software over reports of four crashes where FSD software was engaged in an “area of reduced roadway visibility conditions.” NHTSA said reduced roadway visibility was due to conditions like sun glare, fog, or airborne dust.

NHTSA said in one incident a pedestrian was struck and killed and in another an injury occurred when FSD was engaged in reduced visibility conditions. NHTSA’s preliminary investigation would look into whether FSD’s controls could detect and respond to reduced roadway visibility conditions and if any other similar crashes occurred with FSD engaged, among other inquiries.

NHTSA said the investigation would cover 2.4 million Tesla EVs, which would be every vehicle with the ability to engage FSD in the US.

An FSD investigation regarding visibility issues is a huge concern for Tesla. The automaker uses a vision-only based system for self-driving, which utilizes cameras as its main sensory input. Other automakers incorporate radar, LIDAR (laser), and ultrasonic (sound) sensors to detect objects and read the contours of the road.

Tesla took the vision-based approach that uses machine learning and AI to process information in real time — and make decisions based on visual information. CEO Elon Musk and Tesla believe a vision-based system, with millions of Tesla vehicles on the road collecting data to train AI models, would be the fastest way to implement full self-driving capabilities.

Just last week, Tesla and Musk unveiled its fully autonomous Cybercab, a robotaxi slated for a 2026 production date with a $30,000 price tag. The Cybercab has no pedals or steering wheel and is intended to be used fully autonomously.

Tesla and many bullish analysts see Tesla as more of an AI and tech software company than just an automaker, with the biggest non-car bets being its autonomous software and projects like the Optimus robot.

But any doubt over Tesla’s FSD software capabilities — for instance, reduced functionality in low visibility conditions — is a big blow to the company’s autonomous ambitions.

This isn’t the first investigation into Tesla’s FSD software by the NHTSA. Last year following an investigation, the NHTSA ordered Tesla to perform a software update “recall” of FSD designed to address inattentive drivers.

Tesla is also facing a Department of Justice investigation into whether the company and executives like Musk committed fraud by misleading investors and consumers over FSD’s capabilities.