The National Transportation Safety Board released 500 pages of findings on the death of driver Joshua Brown, 40.
His Model S car collided with a lorry in Florida while in autopilot mode.
It found that in 37 minutes of driving, Mr Brown had his hands on the wheel for just 25 seconds.
The documents also found that Mr Brown had set cruise control at 74mph (119km/h) which was above the 65mph speed limit.
The US authorities investigated Mr Brown's death amid speculation that it might be the first to be caused by self-driving technology. The driver of the truck, which was pulling a trailer, was unhurt.
In its report, the Safety Board said the truck should have been visible to Mr Brown for at least seven seconds before impact but that he took "no braking, steering or other actions to avoid the collision".
The report said that the car remained in autopilot mode for most of his trip and that it gave him a visual warning seven separate times that said "hands required not detected".
In six cases, the system then sounded a chime.
In September, Tesla unveiled improvements to autopilot, adding new limits on hands-off driving. The updated system temporarily prevents drivers from using the system if they do not respond to audible warnings to take back control of the car.
In January, the National Highway Traffic Safety Administration said it had found no evidence of defects in the car.
The Tesla driver involved in a fatal crash in May 2016 was given repeated automated warnings about his driving behaviour, according to a US government report.