Accident investigators from the UK are to join the search for the black box from the Irish coastguard search and rescue helicopter that crashed into the sea off County Mayo on Tuesday.
They will bring specialised equipment to assist the operation.
Rescue teams searching for three crew members, missing after a helicopter crash off the Irish coast, have picked up a signal from a flight recorder.
Captain Dara Fitzpatrick died in hospital after she was recovered.
The signal from the Rescue 116 coastguard helicopter was picked up close to the Blackrock Lighthouse.
The Irish chief Air Investigator Jurgen White said: "It's difficult waters - 40 metres of depth. We need to bring in more equipment to locate the recorder.
"The hope is that the recorder is with the wreckage".
The Irish navy, RNLI, police divers and fishing boats are searching the coast off Mayo.
Captain Dara Fitzpatrick died in hospital after she was recovered from the sea on Tuesday.
The search for chief pilot Mark Duffy and winchmen Paul Ormsby and Ciarán Smith continues.
At the scene: BBC News NI's Julian Fowler
As well as the search on the air and in the sea, this operation is increasingly focusing under the water.
Sonar equipment is deployed and Gardai (Irish police) and naval divers are on standby ready to continue searching for the wreckage under the water.
They're also looking to retrieve the bodies of the missing crew members and also the flight data recorders which may explain why the helicopter disappeared without warning.
The Dublin-based helicopter crew had been assisting colleagues from Sligo in a rescue operation on a UK fishing vessel about 150km (95 miles) west of Eagle Island.
They lost contact at about 01:00 local time on Tuesday on a refuelling journey to Blacksod.
The Irish naval ship, LÉ Eithne, is being used to co-ordinate the ongoing search and rescue operation - the sea element of the search is being run from the ship.
Irish coastguard search and rescue manager, Gerard O'Flynn said: "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."
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'
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.
Irish President Michael D Higgins 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," he said.
The head of the Irish coastguard, Eugene Clonan, described it as a "very dark day" for Irish emergency services.
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 helicopter lost contact on its final approach to Blacksod refuelling depot.
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
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.
Irish coastguard helicopters have been used for a number of operations in Northern Ireland.
Ryan Gray, Senior Maritime Operations Officer at Belfast Coastguard, said last year they were called to aid the hunt for a missing diver in Belfast Lough, and to rescue an injured woman in Strangford Lough, among other incidents.