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Mr Bagley was thrown from his car when it left the road on the Lindis Pass, rolled down a bank and exploded.
Asked about the cause of the crash at the time, police said Mr Bagley's Subaru Impreza apparently missed a corner north of the Cluden Hill summit.
They ruled out speed and alcohol and Sergeant Derek Ealson said ''the vehicle will be examined, but at the time the ... weather was fine, the road was dry, there was little traffic, so none of those factors have played a part''.
Since then, the serious crash unit has investigated and yesterday Sgt Ealson told the Otago Daily Times the cause of the crash appeared to be ''driver error''.
However, exactly what went wrong was uncertain, Sgt Ealson said.
''I don't know if it was a moment's inattention - did he fall asleep, did an animal cross in front?
''There was a dog in the car. Was the dog a factor?
"Fatigue could have been a factor. But because we don't know the [cause], we would just be speculating. I think driver error probably covers that,'' he said.
Mr Bagley was a member of the New Zealand team at the inaugural Youth Olympic Winter Games in Austria in 2012.