DETROIT (AP) — Tesla is letting some drivers to use its Autopilot driver-assist system for extended periods without making them put their hands on the steering wheel, a development that has drawn concern from U.S. safety regulators.

The National Highway Traffic Safety Administration has ordered Tesla to tell the agency how many vehicles got a software update making the change and what its plans are for wider distribution.

In a July letter to Tesla, the agency says it’s concerned that more drivers will activate the change, leading to more driver inattention.

The government has been investigating Autopilot for crashing into emergency vehicles parked on freeways, as well as hitting motorcycles and crossing tractor-trailers.

At least 17 people have died in crashes.