"Based on the facts we have, it was a technical failure. It was not human error," a director of the Norwegian accident investigation board, Kare Halvorsen, told a press conference held in a hangar in the western city of Bergen, where debris from the wreck was visible.
The EC225 Super Puma built by Airbus Helicopters crashed Friday on a small island off Bergen, en route from a North Sea oil platform. The 13 killed included 11 Norwegians, one Briton and one Italian.
While investigators seemed to be in no doubt about the helicopter's technical failure, they did not disclose the exact cause of the accident -- the deadliest helicopter crash in Norway since 1978 -- adding the investigation was still ongoing with French and British assistance.
Eyewitness accounts and footage captured on cell phones indicate that the rotor detached just before the crash.
On Friday, Norwegian and British aviation authorities announced they were grounding the EC225 until further notice.
"We have taken note of the announcements" from Norwegian investigators, a spokesman for Airbus Helicopters, Guillaume Steuer, told AFP.
"They have reported technical causes. That can be several things: conception, production or maintenance," he said.
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On Sunday the company advised all EC225 owners to continue flights as normal in countries where they were authorised.
"The fleet is safe," Steuer reiterated Tuesday.
Older model Super Pumas have been involved in several accidents in the British oil sector, some of them deadly. The most serious dates back to 2009 when a helicopter crashed off of Scotland, killing its 16 occupants after its rotor detached.