Tesla faces a new investigation by the National Highway Traffic Safety Administration, or NHTSA, concerning issues with its “Full Self-Driving” systems, and whether they are safe to use in fog, glaring sun or other “reduced roadway visibility conditions.

NHTSA initiated the investigation after four crashes involving Tesla vehicles with FSD engaged during low-visibility conditions, such as sun glare, fog, or airborne dust. One of these crashes resulted in a pedestrian fatality, while another involved a reported injury.

The Tesla Full Self-Driving semi-autonomous driving system is under the NHTSA's microscope for causing accidents in low-visibility road conditions.