The search operation for the remaining crew of an Irish Coast Guard helicopter that has gone missing has been wound down for the night.
It follows the death of a rescued crew member who has been named as Captain Dara Fitzpatrick.
The 45-year-old had been with the search and rescue service for more than 20 years.
She was one of four crew members who went missing off the coast of Co Mayo, in Ireland, early Tuesday.
Mark Abbey, regional director for CHC, the company which operates the helicopters for the Coast Guard, said: "It is with deepest regret that we can confirm that Captain Dara Fitzpatrick passed away following today's accident off the coast of County Mayo in the early hours of this morning.
"We are devastated by this morning's tragic accident. Our thoughts are with Dara's family and friends, as well as those of the three crew who are still missing.
The Dublin-based Sikorsky S92 helicopter, which was providing cover for another helicopter involved in an early-morning rescue operation, lost contact at around 1am.
Eugene Clonan, acting director of the Irish Coast Guard, said: "Indeed, at this particular point in time, hopes are fading of finding the remainder of the crew."
Debris has been discovered on the surface of the water around one and half miles (2.4km) south-east of Blacksod Lighthouse, which is around six miles (10km) off the coast.
Mr Clonan said it was a "dark day" for Ireland's emergency services."Our thoughts and prayers are with the families at this time," he said.
The Coast Guard chief said contact was last heard from the missing helicopter at around 12.45am.
It had flown directly to the scene from Dublin, travelled around 10 miles (16km) out to sea, then turned back towards land to refuel.
There was no indication that anything was wrong moments before it vanished, with the crew's final transmission: "Shortly landing at Blacksod."Visibility was described as not good at the time.
When the helicopter failed to arrive, a Mayday signal went out and Coast Guard helicopters from Sligo and Shannon along with the Air Corps maritime patrol aircraft Casa were tasked to the scene.
They were joined by lifeboats from Ballyglass and Achill, and five local fishing vessels.
Dublin's Transport Minister, Shane Ross, expressed his "deep concern" over the helicopter's disappearance.
He said: "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, particularly those family members who are awaiting news of their loved ones.
The family of Captain Fitzpatrick who was the mother to a three-year-old son, said they are devastated.
In a statement to Today FM, her family said: "We are so proud of Dara's work and all she has done to save the lives of others over the years.
"We are completely heartbroken and we pray for the recovery of the other three crew."
President of Ireland, Michael D Higgins, paid tribute saying it was a "dark day in the history of the Coast Guard".
"On behalf of the people of Ireland, may I pay tribute to Captain Dara Fitzpatrick who died today," he said.
"We are all grateful for the courage, resolution and exemplary commitment to the aims of the Coast Guard 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."
Mr Higgins said it was also appropriate to acknowledge and pay tribute to the efforts of Ms Fitzpatrick's colleagues at the Coast Guard "who have been working at a time, which must be for them, a time of great grief and uncertainty".
Irish Taoiseach Enda Kenny said: "On behalf of the government I want to issue our deepest sympathy in respect of the family of Captain Dara Fitzpatrick, and the search continues arising from this particular tragedy."
Sinn Fein fisheries spokesman Martin Ferris paid tribute to the "selfless service" of the Coast Guard in saving lives.
"The rescue helicopters are a welcome sight, not just on the deck of a boat in trouble, but also to those injured and stranded on Ireland's mountains," he said.
The Naval Service's LE Roisin ship has arrived at the scene to help in the rescue effort.
Mr Clonan said the missing Sikorsky S92 is one of best helicopters that can be used in search and rescue operations and had a "very good safety record".