WASHINGTON, Jan. 24 (Xinhua) -- The U.S. National Highway Traffic Safety Administration (NHTSA) said Wednesday it is sending investigators to probe a crash in the state of California involving a Tesla sedan in autopilot mode.

A fire truck was struck from behind by a Tesla Model S sedan on a Southern California freeway on Monday. The fire truck was parked in an emergency lane at the side of the highway attending to another accident. The Tesla was driving in autopilot mode, according to the driver.

The National Transportation Safety Board (NTSB), an independent government agency responsible for civil transportation accident investigation, said Tuesday it had already sent two investigators to the scene to conduct a field investigation.

The probe is the second by the safety agency into a crash involving Tesla's autopilot feature. Tesla, along with other major auto companies, has been seeking federal legislation that would allow quicker deployment of self-driving vehicles.

Tesla said in a statement that autopilot is "intended for use only with a fully attentive driver." The electric car company requires drivers to keep their hands on the steering wheel when the vehicle is in autopilot mode.