Debris has been found on the surface of the water around one-and-a-half miles south-east of Blacksod Lighthouse.
Hopes are fading for the survival of four crew members of an Irish Coast Guard helicopter missing off the west of Ireland, the head of the rescue service said.
One crew member was pulled from the Atlantic, in a critical condition, as an intense search continued for three others off the Co Mayo coast – around six miles (10km) west of Blacksod.
The Dublin-based Sikorsky S92, which was providing cover for another helicopter involved in an early-morning rescue operation, lost contact at around 1am on Tuesday.
Eugene Clonan, acting director of the Irish Coast Guard, confirmed the crew member found in the water at around 7am is fighting for their life.
“We don’t hold out much hope for that person,” he said.
“And 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.
A Coast Guard spokesman said the missing helicopter had been providing cover in the evacuation of a crewman who needed urgent medical attention, from a UK-registered fishing vessel approximately 150 miles (240km) west of Eagle Island in Co Mayo.
“Owing to the distance involved, safety and communication support, known as top cover, was provided by the second Coast Guard helicopter, the Dublin based R116,” he said.
The search operation is being co-ordinated by the Coast Guard Rescue Co-ordination Centre in Malin Head.
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.
“This is an extremely difficult time for all concerned.”
Mr Ross appealed for space to be given to those involved in the search operation to complete their work.
“Once again, I send my utmost support to all those affected,” he added.