The National Highway Traffic Safety Administration escalated its investigation into Tesla's Full Self-Driving technology Wednesday, advancing from a preliminary evaluation to a more comprehensive engineering analysis.

The federal safety agency is examining the software's performance in low-visibility driving conditions following nine crashes involving Tesla vehicles equipped with FSD capabilities. One of the incidents resulted in a fatality.

The engineering analysis represents a significant step up in the investigation process, indicating NHTSA's increased concern about the technology's safety performance. This phase involves more detailed technical scrutiny of the system's design and functionality.

The crashes under review occurred when Tesla's FSD software apparently failed to properly detect poor driving conditions that reduced visibility. The investigation will examine whether the automated driving system adequately responds to challenging environmental factors such as fog, heavy rain, or other conditions that impair visibility.

Tesla's Full Self-Driving feature, despite its name, requires active driver supervision and is classified as a Level 2 driver assistance system. The company has been expanding access to the beta version of the software to more customers in recent years.

The outcome of NHTSA's engineering analysis could potentially lead to a recall or other regulatory action if safety defects are identified in the FSD system.