Survivors have described the terrifying moment a helicopter "lost power" and ditched into the sea off Shetland, an incident that trade unions called "deeply concerning".
Four people died after the helicopter went down in the North Sea, police confirmed as they released the names of the victims.
Duncan Munro, 46, from Bishop Auckland; Sarah Darnley, 45, from Elgin; 59-year-old Gary McCrossan, from Inverness and George Allison, 57, from Winchester died following the incident yesterday evening.
The Super Puma helicopter was carrying 18 people when it ran into trouble near to the remote northern archipelago, close to Sumburgh Airport.
Scotland's First Minister Alex Salmond paid tribute on Saturday to the "brave and hard-working" people involved in the rescue effort.
He added: "Our thoughts at this difficult time are with the families, friends and colleagues of those who lost their lives in this tragic incident."
Pat Rafferty, Scottish secretary of the Unite union said the incident was "deeply concerning".
He added that his immediate thoughts were with the families of those involved.
Rafferty said the rescue of the survivors of the accident was " testimony to the bravery and skills of the rescue services".
He went on: "This brings into sharp focus once again the very precarious nature of the transportation of workers to and from offshore platforms.
"The health and safety of working people is our priority and we will be watching events closely as they happen."
Bob Crow, general secretary of offshore union RMT, said he expected an "outpouring of anger" in response to the latest incident.
"The entire Super Puma fleet must remain grounded until the causes of this latest event are established and dealt with thoroughly to the unions' satisfaction and we will support any member who refuses to board any suspect aircraft type in light of this disaster.
"RMT also demands the lifting of the ban on union access to the offshore workforce which in an infringement of basic rights and makes a mockery of pledges on offshore worker safety."
The Super Puma L2 aircraft went down at 6.20pm last night, around two miles west of Sumburgh airport as it was returning to Shetland from the Borgsten Dolphin platform in the North Sea.
The families of those affected have been informed.
An RNLI spokesman said two of the bodies were recovered by an RNLI lifeboat crew from Lerwick, Shetland.
"The lifeboat crew transported the bodies to Sumburgh and we are liaising with other authorities as things develop, " he said.
"Obviously this is the news that everyone, included our lifeboat volunteers, dreaded - our thoughts and prayers are with the families and loved ones of those concerned.
"We can also confirm that one of our lifeboats has also been involved in reclaiming wreckage from the scene as part of the operation."
The helicopter, flown by two crew members, was carrying 16 passengers from an oil rig to the island when it ditched.
One of the men rescued, Sam Smith, described how the helicopter suddenly lost power and there was "no time to brace", it has been reported.
His mother Amanda Smith told Sky News: "He said (the helicopter) seemed to lose power and there was no time to brace - they just dropped into the sea.
"He was by the window so he was able to escape that way as it rolled over.
"He said he had come off better than a lot of people, were his words.
"It doesn't seem real."
The coastguard said the helicopter's life rafts were found empty and some wreckage from the aircraft has started to wash up at the southern end of Sumburgh.
The helicopter's operator CHC, said it was flying for oil company Total and that the aircraft lost communication as it approached the airport on the southern tip of Shetland's main island.
Fourteen people were taken to the Gilbert Bain Hospital in Lerwick, Shetland, for treatment, including the two crew members, officers said.
Five were discharged and 9 detained overnight for observation or because they were suffering from exposure.
Assistant Chief Constable Malcolm Graham said: "Our sympathies are very much with the families of those affected at this difficult time. All families have been informed and specially-trained family liaison officers are currently providing them with support.
"This incident has resulted in a large-scale response from a number of different agencies who have worked closely together to deliver a swift rescue operation.
"We have been able to deploy a number of officers with specialist disciplines from across Police Scotland to assist with the operation.
"We will now be carrying out an investigation to establish the circumstances in due course."
Two lifeboats from Lerwick and Aith were being helped at the scene by helicopters from the coastguard, RAF Lossiemouth and two Bond rescue helicopters.
A Northlink ferry carrying 201 passengers between Shetland and Orkney was also diverted to the scene to help.
Jim Nicholson, RNLI rescue co-ordinator, said he understands two of the bodies were recovered in the area where the helicopter crashed.
"The bodies came to the surface close to the helicopter wreckage," he said.
"The helicopter was in a pretty inaccessible place but the lifeboat crew were able to get to them using an inflatable craft.
"It's fortunate there were not more casualties in a helicopter crash of this kind.
"There appears to have been a catastrophic loss of power which meant the helicopter suddenly dropped into the sea without any opportunity to make a controlled landing."
He praised the efforts of the rescue agencies involved. "I think it's been a very long night and I think the crew have been tremendous."
Scottish Secretary Michael Moore also said his thoughts were with the family and friends of those who lost their lives and were injured.
He said: "There have been too many reminders in recent years of the dangers involved in our offshore oil and gas industry. The bravery and dedication of those in the industry is never taken for granted and we owe them a debt of gratitude.
"We must find out what caused this tragic accident and an investigation is now underway."Suggest a correction