Norwegian investigators say last week's helicopter accident in western Norway that killed 13 people was caused by technical failure and not human error.

The Accident Investigation Board said Tuesday that their probe and visual evidence, including video of the helicopter's rotor propelling into the sea moments before it crashed, indicated sudden mechanical failure. It did not elaborate, saying a full investigation would take time.

The victims — 11 Norwegians, one Briton and one Italian — were aboard the Airbus EC-225 helicopter that was flying from an offshore oil rig in the North Sea when it crashed on the island of Turoey on Friday.

Police have identified the victims, but have not released all the names in accordance with the wishes of some the families.