Initial findings published by the Air Accident Investigation Branch (AAIB), which is leading the inquiry into the tragedy at the Clutha bar in Glasgow on November 29, said there was nothing to indicate engine failure had caused the helicopter to fall from the sky.
The pilot and two police constables on board the twin-engined Eurocopter EC135 were killed when it plummeted at 10.25pm. Six people inside the bar also died.
In its preliminary report, the AAIB said initial examination of the wreckage "provided no evidence of major mechanical disruption of either engine", and there was no apparent reason for the rotor blades to have cut out, although investigators say they were definitely not turning at the moment of impact.
There was no evidence of gearboxfailure, although the report added the force of the collision had caused a shaft carrying the power supply between the rotors and one of the engines to break off, preventing investigators at this stage from ruling out a gearbox problem.
The AAIB stated the helicopter was carrying 95 litres of fuel at the time of the crash and radar contact was lost at 10.22pm - four minutes after pilot David Traill had requested clearance from air traffic control to re-enter the Glasgow Control Zone and return to base.
Helen Krasner, an aviation writer and former helicopter instructor, said the fact the rotor blades had ceased turning pointed to "catastrophic mechanical failure". But, with initial findings indicating this was not the case, she said the cause was "a complete mystery".
David Learmont, operations and safety editor of aviation website Flight Global, said: "I do not believe it was pilot error. This was an extremely experienced pilot operating for a very experienced contractor."
He added that it was "surprising" there was no fire with the amount of fuel that was on board. He said: "For a fire to happen fuel vapour needs to come into contact with heat. So, in this case, fuel vapour did not come in contact with hot engines. The question is why? I don't know."
The AAIB report was published on the day funerals were held for two men who died inside the pub. Mark O'Prey, 44, was laid to rest in a service in St Bride's Roman Catholic Church in East Kilbride, South Lanarkshire, while Gary Arthur, 48, was remembered at a humanist service at Woodside Crematorium in Paisley, Renfrewshire.
The British Airline Pilots' Association (Balpa), which represents plane and helicopter pilots, said the AAIB report was a "helpful first assessment of the facts of the case".
"Unfortunately, at this stage, the reasons behind the crash are, in truth, far from clear," it said. "The AAIB must be given the time to complete its thorough investigation and we have faith in its ability to do so."
The helicopter left its base on the River Clyde at 8.45pm, travelling to Dalkeith in Midlothian before returning to Glasgow. No mayday call was issued before the aircraft went down.
Jim Morris, a former RAF pilot and partner in Irwin Mitchell's Aviation Law team, said the AAIB's findings underlined the need for flight data recorders and cockpit voice recorders to be fitted as standard in helicopters carrying passengers.
In a statement, Eurocopter said: "Eurocopter continues to support the AAIB in its investigation and will continue to report relevant information as soon as it becomes available.
"Flight safety for the thousands of individuals around the world transported in Eurocopter aircraft is and always will be the company's number one priority. We are committed to doing everything we can to ensure that accidents like this do not happen again."