An aviation safety expert has suggested something "dramatic" suddenly occurred to cause the police helicopter crash in Glasgow.
The pilot would have had either little or no control of his aircraft in the final moments of the flight, said Flight Global's operations and safety editor David Learmount.
But he added that although a witness has described the helicopter as dropping like a stone, there were indications that the pilot might have still had "some ability to fly" before the impact.
Speaking on Sky News, Mr Learmount, who is a pilot, said: "This type of helicopter is sophisticated and robust. It's a very modern aircraft. I think what has happened here is that you have had an aircraft that became either uncontrollable or partially controllable.
"We just don't know how much control the pilot did have in the final seconds of the flight. Something dramatic has probably suddenly occurred - probably some mechanical failure of some kind."
He went on: "Helicopters are very mechanically-complicated devices. You only have to look at them to see that. Helicopters can, though, glide. This one was a two-engined aircraft. If you lose one engine you can fly on the other one and if you lose both engines then you can glide.
"If we listen carefully to what the witnesses have been saying it doesn't sound as if this helicopter was, at the end, able to glide.
"It may be that in the final moments the pilot had some control or was in some way able to reduce the rate of descent. It looks as if the aircraft had some ability to fly although I suspect it was dropping pretty fast - dangerously fast for those on board."
Mr Learmount said he thought that if the helicopter had been completely uncontrollable then the crash could have been "even worse".