OSLO, July 5 (Xinhua) -- A fatigue fracture in one of the gears caused a fatal Airbus helicopter crash in western Norway in 2016 that killed all 13 people on board, Norway's Accident Investigation Board (AIBN) said in its final report on Thursday.

The main rotor suddenly detached from the Airbus EC 225 Super Puma helicopter without warning as it was en route from an oil platform in the North Sea to Bergen, Norway on April 29, 2016, the report said.

Shortly after, the helicopter crashed on a small island near Turoy, northwest of Bergen. Eleven Norwegians, one Briton and one Italian were killed.

"The accident was a result of a fatigue fracture in a second stage planet gear in the epicyclic module of the main rotor gearbox. Cracks initiated from a micro-pit at the surface and developed subsurface to a catastrophic failure without being detected," the report said.

The AIBN noted that there is no connection between the way the crew handled the aircraft and the accident. It has also excluded material nonconformity and mechanical failure, as well as maintenance actions by the helicopter operator.

The agency said there are significant lessons to be learned from this investigation related to "gearbox design, safety assessment, fatigue evaluation, condition monitoring, certification requirements and continued airworthiness of the AS 332 L2 and the EC 225 LP helicopters."

The AIBN issued 12 safety recommendations based on the investigation, it said.