The driver killed while testing an autonomous Tesla car last year ignored multiple "visual or aural" warnings ahead of his fatal crash, says the US-based National Transportation Safety Board (NTSB).
The car's autonomous Autopilot features were "actively engaged and controlling the vehicle", added investigators.
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While the system automatically keeps the vehicle in the correct lane and can carry out an emergency stop, drivers are supposed to hold the wheel at all times in case such an incident occurs.
The NTSB report said the car had given Brown a visual warning seven separate times that said "hands required not detected", says the BBC.
Brown had also set his cruise control 74mph - 14mph more than the speed limit, reports the broadcaster.
Tesla has yet to comment on the findings, but released a software update last September "to help keep drivers from using autopilot if they don't respond to in-vehicle safety warnings", reports ArsTechnica.
It adds that a separate investigation into the crash found there was "no evidence of defects" on the Model S involved.
Tesla issued a statement after the crash saying Autopilot was a new technology and in a "public beta phase" at the time.
It added that the system "is an assist feature that requires you to keep your hands on the steering wheel at all times" and that drivers "need to maintain control and responsibility" of the car.