A technical error was almost certainly behind the helicopter crash in which 13 people died on Friday, Norway's Accident Investigation Board says.
"On the basis of the facts we have, this involves a technical fault; it isn't human error," the director of the board's aviation department said.
The Airbus H225 Super Puma crashed in the North Sea after picking up passengers from an oil platform.
Thirteen people died - 11 Norwegians, a Briton and an Italian.
Some of the victims' families have chosen not to identify them publicly. The Briton has been named as Iain Stuart, from Laurencekirk in Aberdeenshire.
Footage of the crash showed the helicopter's rotor detached from the body of the aircraft and spinning through the air.
A warning light had come on in the cockpit of the helicopter on two occasions in the days before the crash, resulting in two components being changed.
Airbus (formerly Eurocopter) H225 Super Puma
Widely used in offshore oil and gas industry around the world; more than 220 in service.
But the Super Puma's recent history is blighted by several accidents:
- In 2012, two EC225 Super Puma helicopters ditched into the North Sea in Scotland - one off Aberdeen and another off Shetland. Both incidents were blamed on gearbox problems and all passengers and crew were rescued
- In August 2013, a different model of Super Puma, the AS332 L2, crashed off Shetland, killing four people
- In April 2009, another AS332 L2 crashed off the Scottish coast; 16 men died when the main rotor separated following gearbox failure. It is one of several accidents involving older model Super Pumas
On Tuesday the aviation director of Norway's Accident Investigation Board, Kaare Halvorsen told reporters that they were "as certain as they can be" that a technical problem caused the crash, rather than human error.
Mr Halvorsen confirmed that the crew had no time to send an emergency message before the crash.
He told Norway's VG website that there had been less than one second between the fault appearing and the helicopter's flight recorder coming to a halt. The recorder is thought to have stopped working when the rotor became detached.
But he stressed that the investigation - in which British and French experts are participating - was at an early stage and no conclusions could yet be drawn.
The Super Pumas remain grounded in both Norway and the UK, though their manufacturer, Airbus, said on Monday that it was no longer recommending a blanket ban on their use. It insists the fleet is safe.
The Offshore Co-ordinating Group, which co-ordinates trade union policy and campaigns in the oil and gas sector, released a statement calling for the H225 helicopter to be grounded until the cause of Friday's crash was established.
"Workers in both the Norwegian and UK sectors must have genuine guarantees from regulators before flights in this aircraft can be considered," said group chair Tommy Campbell.
"Statements from manufacturers and operators will not suffice."