Reuters

The US auto safety regulator investigating Tesla`s Autopilot driver assistance system is demanding an explanation for a software change that allows drivers to keep their hands off the wheel for longer, risking collisions, documents released on Tuesday showed.

The National Highway Traffic Safety Administration (NHTSA) ordered Tesla to answer questions about the Autopilot change and produce documents in a so-called special order dated Jul 26. The NHTSA order did not compel Tesla to recall vehicles.

"The resulting relaxation of controls...could lead to greater driver inattention and failure of the driver to properly supervise Autopilot," NHTSA said in its letter to Tesla.

The NHTSA order underscores the challenges regulators face keeping up with safety-critical vehicle technology that can be changed overnight with a software upgrade delivered over the air.

NHTSA asked when the software update was introduced, the number of vehicles affected, Tesla`s reason for installing, and collisions or near-misses involving vehicles with the software.

"NHTSA is concerned that this feature was introduced to consumer vehicles and, now that the existence of this feature is known to the public, more drivers may attempt to activate it," the agency wrote.

Musk will soon face a major test of his assertions about the technology`s safety when Tesla defends itself later this year in a trial over allegations that failure of its Autopilot driver assistant feature led to death.

Tesla shares were up 7% in late trading Tuesday, outpacing the broader market.