The driver of a tram which crashed in London, killing seven people, may have momentarily fallen asleep in the seconds before the accident.
The official report into the crash in Croydon, south London, confirms that the accident was caused by a driver failing to apply the brakes in time to negotiate a sharp bend.
It concludes that he may have lost awareness or had what is described as a "microsleep" because of fatigue.
The report also reveals that there was a similar speeding incident less than two weeks before the deadly crash.
Seven passengers died and 69 others were injured in the accident on 9 November 2016. Only one person aboard the tram was unhurt.
The tram should have slowed to 20km per hour to take a left turn into the Sandilands tram stop. Instead, it was travelling at 73km per hour - almost four times the speed limit.
As the tram overturned, survivors reported that they were tossed around like "being inside a washing machine".
The dead and most seriously injured were thrown through shattered windows and were crushed under the tram as it skidded to a halt.
The official report by the Rail Accident Investigation Branch (RAIB) concluded that even though the driver had eight years' experience, and had made the same journey around 700 times in the previous two years, he may have lost awareness because he had little to do in the minute immediately before the crash.
The report says there is no evidence that he was tired because of the shift pattern he was required to work.
A small sign just before the bend indicated that the speed limit was 20km per hour, but the driver braked too late.
The report describes how, on 31 October, a passenger emailed Tram Operations Limited to complain that she had been injured when a driver took the same bend too fast.
The complaint was still being assessed by staff at the time of the fatal accident 10 days later, but the driver involved in the earlier incident told investigators that he was reluctant to report the incident himself in case he was suspended from duty.
Investigators don't believe that reporting of the earlier incident would have prevented the fatal accident because the operator would not have fully realised how close the tram came to overturning on that occasion.
Unlike trains, trams rely heavily on the judgement of drivers to judge their own speed given varying circumstances on their journey.
The report recommends that there should be a mechanism to automatically apply the brakes when a tram is travelling too fast.
It also concludes that the signage was inadequate and need to be improved, and windows and doors should be strengthened. There were no emergency exits and passengers had to escape through the windscreen of the cab.
The report recommends here should be a more open culture of reporting mistakes and learning from them. Signage was improved within days of the accident and before the tramway reopened.
The driver has not been named in the report, but is currently on police bail having been interviewed on suspicion of manslaughter.