March 19 (Reuters) - The ​U.S. auto safety regulator on Thursday deepened a probe into Tesla's driver-assistance system to evaluate ‌its effectiveness in identifying degraded road conditions after nine crashes linked to the technology including one that was fatal.

The National Highway Traffic Safety Administration said its engineering analysis follows an earlier preliminary review and broadens the probe to about 3.2 million ​Tesla vehicles across multiple models equipped with the system, covering most vehicles on U.S. roads.

Tesla shares dropped 2.5% in morning trading.

The probe intensifies ​regulatory scrutiny of the self-driving technology central to Tesla's ambitions to build out a ⁠fleet of robotaxis that would underpin its future, as the company shifts focus away from an ailing ​auto business.

Tesla's so-called Full Self-Driving (FSD) technology relies mainly on cameras rather than a combination of sensors such as radar ​and lidar used by several rivals such as Waymo. It has launched a limited robotaxi service in Austin, Texas, and plans to expand the offering rapidly this year.

NHTSA said on Thursday its probe focuses on Tesla's "degradation detection" system, a feature designed to recognize when ​the vehicle's camera-based technology cannot reliably perceive the road, such as in glare, dust or other airborne ​obstructions, and to warn drivers to take control.

It could lead to a recall or other enforcement action if regulators find a ‌safety ⁠defect. Tesla typically rolls out over-the-air software updates to resolve safety issues.

Tesla did not immediately respond to a Reuters request for comment.

The company's analysis showed that if an update to the degradation detection system was installed at the time of the crash, it may have affected three of the incidents, NHTSA said.

NHTSA said available data ​raised concerns that the system, ​both as originally deployed ⁠and after subsequent updates, may fail to detect reduced visibility or provide adequate warnings.

The agency is expected to examine the performance of the updated system, including when ​it was deployed, how widely it has been rolled out and whether it ​improves the system’s ⁠ability to detect visibility issues and alert drivers in time.

Two of the crashes NHTSA identified also involved injuries.

In several of those crashes, the system did not recognize conditions that impaired camera visibility or issue alerts until just before impact, ⁠limiting the ​driver's ability to respond.

The agency also identified additional crashes including some where ​the system lost track of or failed to detect a vehicle ahead in its path.