By Hyunjoo Jin

(Reuters) -U.S. auto regulator NHTSA has sent a special order to Tesla asking questions about changes to the driver monitoring system for its Autopilot software, documents released on Tuesday showed.

Federal regulators are honing in on the systems Tesla uses to ensure that drivers are paying attention while their vehicle is operating on Autopilot.

In its order dated on July 26, the National Highway Traffic Safety Administration raised concerns about a change by Tesla which allows drivers to use Autopilot for extended periods without prompting the driver to apply torque to the steering wheel.

"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 agency is investigating the performance of Autopilot after identifying more than a dozen crashes in which Tesla vehicles hit stationary emergency vehicles. It is also investigating whether Tesla vehicles adequately ensure drivers are paying attention when using the driver assistance system.

In June 2022, NHTSA upgraded an earlier probe of 830,000 Tesla vehicles into an engineering analysis - a required step before it could potentially demand a recall. Last month, NHTSA sought updated responses and current data from Tesla in the probe.

Autopilot is intended to enable cars to steer, accelerate and brake automatically within their lane, while enhanced Autopilot can assist in changing lanes on highways.

(Reporting by Hyunjoo Jin; editing by Peter Henderson)