Detroit - A Tesla Model S involved in a fatal crash with a semitrailer in Florida March 1 was operating on the company’s semi-autonomous Autopilot system, federal investigators have determined.

The car drove beneath the trailer, killing the driver, in a crash that is strikingly similar to one that happened on the other side of Florida in 2016 that also involved use of Autopilot.

In both cases, neither the driver nor the Autopilot system stopped for the trailers, and the roofs of the cars were sheared off.

The crash, which remains under investigation by the National Transportation Safety Board and the National Highway Traffic Safety Administration, raises questions about the effectiveness of Autopilot, which uses cameras, long-range radar and computers to detect objects in front of the cars to avoid collisions. The system also can keep a car in its lane, change lanes and navigate freeway interchanges.

Tesla has maintained that the system is designed only to assist drivers, who must pay attention at all times and be ready to intervene.

In a preliminary report on the March 1 crash, the NTSB said that preliminary data and video from the Tesla show that the driver turned on Autopilot about 10 seconds before the crash on a divided highway with turn lanes in the median. From less than eight seconds until the time of the crash, the driver’s hands were not detected on the steering wheel, the NTSB report stated.

“Neither the preliminary data nor the videos indicate that the driver or the ADAS (Advanced Driver Assist System) executed evasive maneuvers,” the report stated.