An Irish coastguard helicopter pilot has died following a crash off the coast of Mayo in the Republic of Ireland.
Captain Dara Fitzpatrick was taken to hospital on Tuesday after being found in a critical condition by RNLI crew.
The surface search for her three colleagues who are still missing will continue overnight.
The air search, however, will be scaled down until first light on Friday.
The aircraft lost contact at about 01:00 local time as it was coming in to refuel during a rescue operation.
The three other crew on board have been named as Chief Pilot Mark Duffy and winchmen Paul Ormsby and Ciarán Smith.
Irish coastguard search and rescue manager, Gerard O'Flynn said Ms Fitzpatrick was the most senior pilot with CHC which runs the contract to provide search and rescue services in the Republic of Ireland.
"She's been with the company for some 20 years and outside of her work as a pilot she did an enormous amount of work on water safety and was always available to do school visits and just highlight basic water safety," he said.
'Very dark day'
"For all of us involved in the coastguard and particularly, for her family and everybody, it does come as a complete shock, and we want to extend our sincere sympathy to all her family and indeed to her flying colleagues in CHC and simply to everybody who knew her."
Capt Fitzpatrick was the mother of a young son. Her sister, Niamh Fitzpatrick, paid tribute to her on Twitter and asked for prayers for her missing crew members.
Mr O'Flynn said the Irish naval service, the Garda (police) diving unit and the Royal National Lifeboat Institution (RNLI) had joined the coastguard's "hugely resourced" search operation.
He also said that a number of fishing vessels were assisting the emergency services and were "doing Trojan work out there".
"Obviously with the passage of time the chances of recovering somebody alive decreases, but we are treating this as a search operation and we don't give up hope until we absolutely have to."
At the scene: BBC News NI's Julian Fowler
I spoke to one of those who had been out and he described the conditions as quite rough with a four to five metre swell but said they had recovered some of the wreckage.
He said that the debris field covered quite a considerable area.
But the search is continuing for those three missing crew members as well as the flight recorders which may go some way to explaining what actually happened here.
The head of the Irish coastguard, Eugene Clonan, told a news conference earlier on Tuesday that it was a "very dark day" for emergency services in Ireland.
He added that as time went on, "hopes are fading that we will find the rest of the crew".
Debris has been spotted in the water and a search by two RNLI lifeboats, two Irish coastguard helicopters and several local fishing boats is focusing north of Achill Island in County Mayo.
The helicopter which crashed was a Dublin-based aircraft that had been providing support for another Sligo-based coastguard helicopter during an overnight rescue mission.
The Sligo helicopter had been responding to a distress call from a crewman in need of medical attention on a UK fishing vessel about 150km west of Eagle Island.
The BBC understands that the second helicopter, the R116 from Dublin, was sent to help the Sligo aircraft communicate with the base at Malin, as the fishing boat was too far out for the first helicopter to stay within communication range.
The Sligo crew picked up the fisherman, who had a serious hand injury, and transferred him to hospital.
Shortly afterwards, communication was lost with the Dublin-based aircraft.
Conditions were described as good when the R116 helicopter - crewed by two pilots, a winch man and a winch operator - lost contact on its final approach to Blacksod refuelling depot.
The Sligo helicopter returned to the scene and began to search for their colleagues.
Declan Geoghegan from the Irish coastguard said the accident had hit the "rescue family" hard.
Irish coastguard helicopters
- Five Sikorsky search-and-rescue helicopters deployed at bases in Dublin, Waterford, Shannon and Sligo
- Respond to emergencies at sea, inland waterways, offshore islands and mountains across island of Ireland
- Also used during flooding, major inland emergencies, intra-hospital transfers and aerial surveillance
- Handle about 2,500 marine emergencies each year
"We don't know what happened and obviously there will be an air accident investigation into it," he said.
"We'll continue to search for the missing crew members until such time as we have recovered them and the flight recorder."
The Irish President, Michael D Higgins, paid tribute to Cpt Fitzpatrick on "behalf of the people of Ireland".
He said she had lost her life "while providing assistance to others".
"We are all grateful for the courage, resolution and exemplary commitment to the aims of the coastguard that Captain Fitzpatrick and her colleagues have consistently displayed.
"My thoughts are with her family at this difficult moment and also with the families of the missing crew," the president said.
The Irish Minister for Transport, Tourism and Sport, Shane Ross, also offered his condolences, saying: "As the search for the Dublin-based helicopter R116 is currently under way I would like to express my sincere support and sympathies for all those involved."
The search operation is being led by the Coast Guard Rescue Coordination Centre in Malin Head, County Donegal.
The Irish naval vessel, the LÉ Róisín, arrived in the search zone to assist on Tuesday morning.
It is the second fatal accident involving Irish Coastguard crew members over the past six months.
In September 2016, volunteer Coastguard Caitriona Lucas died while assisting in a rescue operation off the County Clare coast.
The mother of two was one of three crew members on board a rigid inflatable boat which flipped over during a search for a man near cliffs at Kilkee.