Joshua Brown's Tesla warned him seven times to put his hands back on the wheel before he plowed into a truck.
A National Transportation Safety Board report on the deadly crash also found that Brown had his hands on the wheel of the Tesla Model S for 25 seconds out of 37 minutes that the car was on autopilot.
The crash, near Gainesville, Florida, in May 2016, drew attention because of the questions it raised about the safety of self-driving cars.
An earlier investigation by the National Highway Traffic Safety Administration found that the crash was not the result of any defect in Tesla's autopilot feature, which can keep a car in a lane and brake to avoid traffic and other obstacles.
In this case, the car hit the trailer of a truck that pulled across its lanes of traffic at an intersection.
The NTSB cautioned that its report was only fact-finding about the crash. The agency said it was not drawing conclusions about why the crash happened.