(CNN) -- A helicopter crash in the North Sea off Scotland's Shetland islands killed four people, officials said Saturday.
Fourteen people have been rescued following the crash Friday, Police Scotland 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," said Malcolm Graham, assistant chief constable of Police Scotland.
There were 18 people aboard; 16 passengers and two crew members. It went down Friday evening, police said
Shetland maritime officials first got word from a Scottish air rescue coordination unit that they had lost contact with a Superpuma helicopter, said Sophie Turner of the United Kingdom's Maritime and Coastguard Agency.
Authorities learned later that the aircraft ditched about two miles west of Sumburgh Airport in the Shetland isles, according to Turner.
Most of the passengers are workers on the Borstein Dolphin oil and rig platform, the spokeswoman said.
Nationalities were not immediately known.
Skies were misty with fog when the aircraft went down -- conditions Turner said are not unusual for that area. She added that authorities had yet to identify the cause of the crash.
The Superpuma is a twin-engine helicopter that is often used to transport passengers given its relatively large cabin, according to its manufacturer, Eurocopter.