updated 10/21/2013 7:37:47 AM ET 2013-10-21T11:37:47

LONDON (Reuters) - A British air safety investigation team said on Thursday that it had found no evidence of technical failure in a helicopter that crashed in the North Sea near Scotland's Shetland Islands.

Four oil rig contractors were killed when a Super Puma L2, made by EADS' unit Eurocopter, crashed into the sea off Shetland's rocky southern coast on August 23.

The helicopter, carrying 16 passengers and two crew, was operated by CHC Helicopter for France's Total, and was heading to Sumburgh airport in Shetland.

"To date, no evidence of a causal technical failure has been identified," the Air Accident Investigation Branch (AAIB) said in a report, adding that detailed examination of the black box data and the helicopter was continuing.

The investigators said preliminary analysis of the data showed the helicopter had been on the normal flight path to the airport when it suddenly started to descend faster and lower than the normal approach two miles from the runway.

Both engines had been delivering power until the helicopter, which was intact, hit the water, it said.

The AAIB report, known as a special bulletin, comes after a British helicopter safety group last week ended a six-day suspension on all flights by Super Puma helicopters in the North Sea that was imposed following the crash.

The crash was the fifth accident in four years in the area involving different models of Super Pumas which included a fatal crash of an L2 in April 2009 in which 16 people were killed.

A spokesman for the Unite union said the AAIB report did not alleviate offshore workers' fears about the safety of the Super Puma fleet and the industry had to take action on offshore helicopter safety.

"It's not just about whether the Super Puma fleet is airworthy, more urgently it's a question of whether they are fit for the safe transportation of people to and from offshore installations," said the spokesman.

(This story is refiled to correct "casual" to "causal" in 4th paragraph)

(Reporting by Brenda Goh, Editing by Belinda Goldsmith)

(c) Copyright Thomson Reuters 2013. Check for restrictions at: http://about.reuters.com/fulllegal.asp

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