Rescuers have admitted that the chances of finding any survivors of a helicopter crash in the North Sea off the Norfolk coast are slim.
Five people have already been confirmed dead and their bodies returned to Great Yarmouth.
A massive search and rescue operation is continuing for a further six who had been on board the Sikorsky S76 when it went down on Tuesday night.
However as time goes on, the likelihood of finding anyone left alive is disappearing.
Clive Mather, chairman of Shell UK, which chartered the helicopter, said: "The longer we continue without finding them our hopes are fading."
He described it as a "terrible accident" and said an investigation team was being assembled.
The helicopter was on a routine trip from Norwich airport to rigs in the North Sea.
It left Norwich at 1900 BST on Tuesday with nine workers and two crew on board, flew to the Clipper platform, a gas rig in the North Sea, and then continued to the Santa Fe Monarch gas drilling rig.
It was on this leg of the journey that the helicopter crashed, two miles from the rig and 25 miles north-east of Great Yarmouth.
Staff on the Santa Fe Monarch rig made the call to the coastguard saying the helicopter had gone down.
Keith Chanter, chief executive of Bristow, which operated the aircraft, said: "As it approached the rig, the aircraft inexplicably ditched in the sea."
An RAF Sea King from Wattisham in Suffolk was scrambled and five bodies were recovered from the water within two hours.
The Sea King was stood down in the early hours of Wednesday morning and the search was continued by coastguards and rig support vessels and lifeboats that had been in the area.
They are concentrating on a five-mile radius of sea close to the rig although no wreckage from the helicopter has been spotted.
The helicopter was owned by the firm Bristow and was chartered by Shell Expro.
Mr Mather said the helicopters were through "rigorous maintenance schedules" and had a "very good" safety record.
The company has suspended all helicopter flights to and from the rigs in the southern North Sea.
Bristow said the helicopter was fully fitted with safety equipment including flotation bags to hold it afloat long enough for people to get out.
Weather conditions were good in the area at the time of the crash and overnight and investigations into the incident are likely to focus on mechanical failure.
Tom Botts, managing director of Shell Expro, said: "We are in the process of taking statements from the staff that were actually on the platform and we believe some people will have seen the helicopter ditch."
A spokesman for RAF Kinloss in north Scotland, where the operation was co-ordinated, said there was "good visibility and light winds" at the time of the crash.
Steve Finlay, duty rescue controller, said the search continued on Wednesday under "near perfect conditions".
He said: "We still have not found survivors and we fear that this points to a worst
All the people on board would also have been wearing survival suits - standard practice for travelling to the rigs.
The two crew on board worked for Bristow, three of the passengers were Shell staff, three worked for engineering firm Amec and two for Amec subcontractors.
The remaining passenger was from Oil Field Medical Services.
Cleveland Police said two of the dead men were from the Teesside area.
Northern Constabulary in Scotland confirmed that one of the people missing in the crash is from Dingwall in Ross-shire.
The one S76 helicopter operated by Bristow out of Aberdeen has been grounded.
Four inspectors from the Air Accidents Investigation Branch (AAIB) are at the crash scene carrying out an inquiry which includes the search for wreckage using what is known as side-scan radar.
A full AAIB report will follow the investigation and it is possible that the branch could also publish an interim report.
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