A report by the National Transportation Safety Board has found that Tesla was not at fault for a fatal crash last year, which killed a former US Navy SEAL after his Model S sedan collided with a truck while driving in autopilot mode.

The 538-page report could not state the cause of the crash, which killed Joshua Brown near Williston, Florida. 

In 2016, the automobile manufacturer said that when a vehicle is on autopilot it 'does not allow the driver to abdicate responsibility'.

Earlier, media reports alleged that Brown was watching a film when the crash occured, which his family lawyer Jack Landskroner refuted as being 'unequivocally false'.

The Brown family has not filed a lawsuit against Tesla and was reviewing the report. 

However, the crash has raised questions about the safety of systems that can perform driving with little intervention from passengers but can also not fully replace people as drivers.

System performance data downloaded from Tesla indicated that Brown switched on automated vehicle control solutions Traffic-Aware Cruise Control and Autosteer lane keeping system when driving the vehicle.

According to the NTSB report, the autopilot mode was on for most of Brown's 37-minute journey. The car’s system had given him visual warning seven times, stating 'Hands Required Not Detected'. 

"According to the NTSB report, the autopilot mode was on for most of Brown's 37-minute journey."

In six out of these seven instances, the system also sounded an audible warning before it returned to 'Hands Required Detected' for one to three second periods.

The report stated that though Brown was alerted to keep his hands on the wheel, information indicated it to have been for just 25 seconds. 

Last September, Tesla made improvements in autopilot and added new restrictions on autopilot and other features. The improved system prevents drivers from using the features if they do not respond to audible warnings.  

The NTSB makes safety recommendations but cannot order recalls. It released the preliminary report of the crash on 26 July 2016. 

Image: Tesla Model S following its recovery from the crash scene near Williston, Florida. Photo: courtesy of Florida Highway Patrol investigators.