Federal regulators said Monday that the driver of a Tesla Model S killed in a collision while the car was in autopilot mode did not have his hands on the steering wheel for a prolonged period of time. He was repeatedly warned by the vehicle that his hands were necessary, the regulators said.

That's one of the findings contained in documents that the National Transportation Safety Board (NTSB) is releasing as part of its ongoing probe into the death of Joshua Brown. The motorist from Ohio was killed last year in a Florida highway crash when the Tesla he was driving struck a tractor-trailer (PDF) as the semi was crossing an intersection of a divided highway that did not have a traffic signal. The crash raised eyebrows about the safety of new automated driving features when used during long stretches of driving. It was also the nation's first crash fatality involving a vehicle in self-driving mode.

Tesla's autopilot mode allows a vehicle to maintain the speed of traffic, and an auto-steer function is designed to help keep the Tesla inside its lane. The board said the Tesla alerted the driver seven times with a visual of "Hands Required Not Detected." The authorities said the motorist, a former Navy Seal, had his hands on the wheel for 25 seconds during the 37 minutes of the trip when they should have been placed on the steering wheel. That's according to "system performance data" from Tesla, the government said.

Tesla has not responded to the new documents the board released, but it has said that motorists cannot "abdicate responsibility" when they're in the driver's seat. Following the May 2016 accident, Tesla announced a new update in September to help keep drivers from using autopilot if they don't respond to in-vehicle safety warnings.

Meanwhile, in a separate investigation into the crash, the US National Highway Traffic Safety Administration in January found no evidence of defects. The administration announced it would not order a recall and said Brown had ignored the in-vehicle warnings telling him he needed to keep his hands on the wheel.

In the latest regulatory documents on the incident, the National Traffic Safety Board disputed some accounts that Brown was watching a Harry Potter movie during the crash last year. The board said it found several electronic devices, but there was no evidence that they were being operated during the accident.

The documents, which include a crash reconstruction report, encompass various aspects of the investigation, including highway design, vehicle performance, and human performance.

The Brown family has not sued Tesla over the crash. Brown family attorney Jack Landskroner told Reuters that he was reviewing the latest documents the NTSB released as part of its ongoing investigation into the crash. The board said the documents amount to "factual information collected by NTSB investigators."

The board's official report on the incident will be released at an undefined date, the NTSB said.