Investigators from the National Transportation Safety Board (NTSB) and the National Highway Traffic Safety Administration were sent to the area of the crash on Wednesday, in Culver City, California.
Tesla said in a statement that Autopilot is "intended for use only with a fully attentive driver". "Amazingly there were no injuries".
Tesla's Autopilot system was in the spotlight after a fatal crash involving a Model S. Last year, the chairman of the NTSB said the car's operational limitations had played a major role in the fatal outcome of the crash, during which the vehicle had been in Autopilot mode.
Watch Tesla's stock price move in real time here. The Tesla S was apparently set in its self-driving mode and traveling at approximately 65 miles per hour when it ploughed right into the back of a big red fire truck owned by the Culver City Fire Department. As Autopilot technology continues to be developed, more advanced functionality will likely be made available to Tesla owners over time nearing full self-driving capabilities.
The California Highway Patrol and Culver City Fire Department told the San Jose Mercury News that the EV had rear-ended the truck, which was blocking the emergency or carpool lane at the scene of a previous accident.
Tesla shares slipped 2% in Wednesday´s trading session amid reports that investigators were probing a crash on a Californian motorway that could be linked to its self-driving technology. As a result, the car's auto-braking never activated and it slammed full speed into the side of the truck. Tesla has taken steps to prevent drivers from using Autopilot improperly, including measuring the amount of torque applied to the steering wheel and sending visual and audio warnings.
The NTSB has previously said Tesla's Autopilot system was a contributing factor in a 2016 fatal crash in Florida.
The board said it was still gathering information about the incident, namely, whether the "Autopilot" feature was engaged when the accident took place and whether the driver was ignoring any warnings. The vehicle crashed into the back of a fire truck on the freeway Monday, but the driver wasn't injured, authorities said.
Separately, NHTSA's Special Crash Investigations inquiry into the 2016 crash concluded that multiple driver-assist and crash avoidance systems had limitations but did not respond to the impending collision. To its credit, Tesla has included safety features that warn drivers to keep their hands on the wheel and pay attention to the road.