Crash rates for Tesla vehicles have plummeted 40% since Autopilot was first installed, according to a government report released on Thursday.

The National Highway Traffic Safety Administration released the report after closing its six-month investigation into Tesla's first known Autopilot fatality. NHTSA did not find a safety defect in Tesla's Autopilot and is not issuing a recall.

The report found that since Autosteer was installed in Tesla vehicles in 2015, Tesla crash rates have dropped by almost 40%. Autosteer is a flagship feature of Autopilot and keeps the car in its lane even when approaching curves.

Tesla provided the Office of Defects Investigation with mileage and airbag deployment data for all 2014-2016 Model S and Model X vehicles equipped with Autopilot technology. The investigators used this data to calculate crash rates prior to and after Autopilot installation.

It's good news for Tesla Autopilot, which has come under scrutiny in the past few months following the fatal accident in May. Joshua Brown, 40, was driving his 2015 Model S with Autopilot activated when he crashed into a trailer making a left turn in front of his car.

Tesla wrote in a blog post following the May accident that the Autopilot system did not notice "the white side of the tractor trailer against a brightly lit sky, so the brake was not applied."

The report said there was no Tesla Autopilot defect that caused the accident to occur. It found that Tesla Autopilot should not have been expected to detect traffic crossing in front of it and that there was ample time (7 seconds) for the driver to hit the brake before hitting the truck.