What we know so far:

Captain Dara Fitzpatrick, who was recovered critically ill this morning, later died in hospital. She is survived by her three-year-old son;

The three missing crew members have been named locally as Mark Duffy, Ciarán Smith and Paul Ormsby;

A search operation is on-going for the missing Coast Guard helicopter and crew members; Contact was lost at about 12.45am this morning;

The aerial search will resume at 7am tomorrow, Wednesday. Crews hope to continue the surface search through the night tonight;

The rescue helicopter's last known location was about six miles west of Blacksod, Co Mayo;

Debris was found during today's search. The black box has not yet been located;

The director of the Coast Guard said "hope is fading" for the missing crew members.

Latest 11pm: Naval vessels the LÉ Roisin and the LÉ Eithne are continuing the search tonight for the three missing crew members of Rescue 116 off Mayo's north coast.

Gerard O'Flynn from the Irish Coastguard says the naval service is playing a vital role in the search operation.

"The air side of it is being scaled down for obvious reasons, with reduced visability, but the surface search will continue," he said.

"The LÉ Roisin is at anchor and is continuing to search. The LÉ Eithne is also joining it, so that element of the search will continue throughout the night."

We are devastated with the tragic news of #Rescue116 and are beyond words. Thank you for the outpouring of support. Go Mairidis Beo #LODD — Rescue 117 (@R117Waterford) March 14, 2017

Captain Dara Fitzpatrick. Niall Carson/PA Wire

Update 6.15pm: The family of Captain Dara Fitzpatrick have confirmed she is survived by her three-year-old son, her three sisters, brother and her parents.

They told Today FM: "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."

Earlier, her sister Niamh paid tribute to her over twitter.

My brave sister Capt Dara Fitzpatrick lost her life in #Rescue116 crash. We are devastated. Please pray for recovery of 3 remaining crew. — Niamh Fitzpatrick (@NFitzPsychology) March 14, 2017

Update 5pm: President Michael D Higgins has paid tribute to Captain Dara Fitzpatrick and the three missing crew members of Rescue 116.

Captain Fitzpatrick was 45 and had been with the search and rescue service for more than 20 years.

Hopes are fading for the survival of the other three crew members, who have yet to be found, acting head of Coast Guard Eugene Clonan said.

The aircraft lost contact at around 12.45am on Tuesday just before it was about to land to refuel, giving no indication of anything wrong.

"Today marks a dark day in the history of the Coast Guard / Garda Cósta na hÉireann, with a member of this important service losing her life while providing assistance to others," President Higgins said.

"On behalf of the people of Ireland, may I pay tribute to Captain Dara Fitzpatrick who died today...My thoughts are with her family at this difficult moment and also with the families of the missing crew."

Capt Fitzpatrick was one of two female Coast Guard pilots who made Irish aviation history three years ago by flying the first all-woman mission for the service.

The pair flew a cardiac patient from west Co Cork to Cork University Hospital before transferring a critically-ill five-year-old child from the hospital to Temple Street Children's Hospital in Dublin.

Dara Fitzpatrick talks about the demands of her role as captain, and her passion for the job, in this video originally recorded to promote a balanced attitude to alcohol.

Some give some, but some give all- As light fades tonight a poignant message on this terrible day#Rescue116 #IrishCoastGuard #999family pic.twitter.com/Z64JoJdy9I — Event Fire Solutions (@fire_event) March 14, 2017

Update: 4.40pm: Taoiseach Enda Kenny has offered his "deepest sympathy" to the families of Irish coast guard workers involved in the tragic circumstances off the coast of Mayo on Monday night, writes Fiachra Ó Cionnaith, Political Correspondent in Washington DC

Speaking in Washington DC where he is currently on a week-long St Patrick's Day trade mission to the US, Mr Kenny said the tragedy happened to "exceptionally professional" personnel and that the investigation must "uncover" what went wrong.

Irish Taoiseach Enda Kenny speaking to journalists at his hotel in Washington. Photo: Niall Carson/PA Wire

He said the confirmed death of captain Dara Fitzpatrick was tragic, and stressed the "search and rescue" operation for the remaining three coast guard workers is continuing.

"On behalf of 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.

"I know this area well, off the coast of black sod, and obviously the rescue agencies are now co-ordinating the search in the locality, assisted by the Ballyglass lifeboat, the Achill lifeboat and local fishermen."

"On behalf of Government again, we pay our respects on the family of Dara Fitzpatrick and the search continues for other crew members.

"Clearly the analysis and investigation into this will want to uncover what exactly happened in respect of the lack of communications, loss of contact with rescue helicopter 116," he said.

Mr Kenny was speaking shortly after the coast guard confirmed that captain Dara Fitzpatrick had died despite the best efforts of medics at Mayo General Hospital.

She was transferred to the facility after being found in a critical condition due to the accident on Monday night.

Captain Dara Fitzpatrick

Recovering the black box

Safety checks were carried out on the disappeared Sikorsky S92 helicopter and four others in January after its maker issued an alert.

No issues were found with any of the Irish Coast Guard helicopters when precautionary inspections were ordered after an incident involving the same make and model on a North Sea platform late last year.

It was landing on the West Franklin rig on December 28 when it left significant gouge marks on the deck.

Gerard O'Flynn, operations manager with the Irish Coast Guard, said in January that the inspections of the tail rotor had not raised any issues.

"All operators were required to carry out pretty well immediate inspections. As of now, all the helicopters are fully operational," he said.

Helicopters stationed at the Waterford and Shannon bases were checked first, followed by Dublin and Sligo.

The aircraft are owned and operated by a private company called CHC Ireland, under contract with the Department of Transport for the Irish Coast Guard.

They carry out all air search and rescue missions off the Irish coast under the contract over the past six years.

Air accident investigators say recovering the missing Coast Guard helicopter's black box could be vital to solving what happened the aircraft.

The Sikorsky S92 - one of four in regular operation by the search and rescue service, with another on standby - was the last one acquired in an upgrade of the fleet completed three years ago.

Since then it has been central in hundreds of successful missions off the Irish coast, including casualty evacuations and long range operations.

Last year alone, the fleet was involved in 886 missions - the longest being at a range of 150 miles west of Loop Head, Co Clare, last March.

Long-range missions

The Sikorsky S92s - which replaced the old S61 models - are manufactured to allow for long-range missions of up to 365 nautical miles and improved cruising speeds.

They are also equipped with enhanced technology, including forward-looking infrared low-light cameras, night sun searchlights, ambulance level paramedic care facilities and satellite communications.

They can hover in strong crosswinds or tailwinds of up to 35 knots.

Eugene Clonan, acting director of the Irish Coast Guard, said they are "fairly new helicopters and very modern".

"This helicopter has all the safety equipment onboard, with duplicate communications, flotation bags," he said.

"The crew themselves would have immersion suits, life jackets, helmets and would have training for these types of emergencies."

The immersion suits are designed to help crew survive for some time in the water.

Mystery surrounds what happened the aircraft in its final moments, with no indication of any problems in its final communications with the Coast Guard Rescue Coordination Centre in Malin Head.

Just before a planned landing on the Co Mayo coastline to refuel, the last transmission was: "Shortly landing at Blacksod."

Officals from the Air Accident Investigation Unit have been sent to the scene and will carry out a full investigation.

Jurgen Whyte, Chief Aeronautical Officer with the unit, signalled the helicopter's black box could be crucial to learning what went wrong.

"These recorders are modern enough that we can actually - if we can recover the recorder and if it is functioning correctly - we can hopefully re-analyse the flight and actually get the aircraft to fly again digitally," he said.

"That will help investigators a lot to actually determine what was happening in the final moments of the flight."

Captain Dara Fitzpatrick

Latest 4.30pm: Ministers Shane Ross and Simon Harris have paid tribute to the crew of the coastguard helicopter which went down off the Co Mayo coast this morning, writes Elaine Loughlin, Political Reporter

Transport Minister Shane Ross expressed condolences to the family of Captain Dara Fitzpatrick whose body was recovered but was later pronounced dead.

Mr Ross described her as "a woman of extraordinary courage and professionalism who made the ultimate sacrifice in the service of others, a young mother whose loss will be hugely felt."

The search is still ongoing for the three other crew members of the search and rescue helicopter.

Mr Ross said: "Luckily there are rare individuals among us who dedicate their lives to the service of others some even do so to enormous risk to themselves, to their health and even to their very lives.

"Today is one of the darkest days suffered by our brave committed emergency services, the news this morning that contact was lost with one of our coast guard helicopters carrying four crew members is shocking.

He said his thoughts and prayers are with the families and friends of crew members.

"Currently a major search is being carried out.

"The government is determined that no resources will be spared in the search for the missing crew," he said, adding that the LE Roisin arrived in the search zone this morning.

Minister for Health Simon Harris said it was "striking" that "on a bright spring day such a dark cloud hangs over Ireland".

He said the crew of Rescue 116 represent all that is good about our country and added that they epitomised "courage, bravery, selflessness and dedication to the welfare of others".

Mr Harris said: "Often at times of great difficulty and sadness, we see people pull together - this has been so evident today as the ongoing, collective effort of our emergency services and the local community work together on this search mission.

"Our hearts go out to the families of the crew, their colleagues, their communities and all in the Emergency Services community at this time of great sorrow."

Latest 4.20pm: The sister of Captain Dara Fitzpatrick has paid tribute to her on Twitter.

Niamh Fitzpatrick from Dublin says the family are 'devastated' after the loss of Dara in the Rescue 116 crash.

Niamh added that she is praying the three other members of the crew will be recovered soon.

Earlier 2.30pm:. The crew member recovered after this morning’s helicopter crash has died, the Irish Coast Guard has confirmed.

The deceased Coastguard member has been named as Captain Dara Fitzpatrick who was a senior pilot with the coastguard with 20 years experience.

At a briefing at Blacksod lighthouse the Search and Rescue operations manager with the Irish Coastguard, Gerard O’Flynn, said the news was a hammer blow.

Captain Fitzpatrick had been transferred to Mayo University Hospital after being found in a critical condition.

She was a well known member of the Coast Guard service and was also involved in several water safety campaigns.

More to follow ...

Update 1.20pm: The Garda Water Unit has arrived at the pier beside Blacksod Lighthouse to assist the search.

Five Coast Guard jeeps are also at the scene.

A Department of Transport representative said: "All possible assets have been deployed to the scene".

The Taoiseach Enda Kenny who is on a visit to the United States is being briefed on the mission in Blacksod.

In a statement he says his thoughts are with the families of the crew and the emergency services working in the area.

Local fishermen and members of the local community return to Blacksod, Co Mayo, as the search continues for an Irish Coast Guard helicopter.

Update 11.40am: Pieces of debris are being recovered by vessels helping in today’s operation.

Jurgen Whyte is the Chief Aeronautical Officer with the Air Accident Investigation Unit, he says they are hopeful the helicopter’s black box will tell them what went wrong.

"These recorders are modern enough that we can actually, if we can recover the recorder and if it’s functioning correctly, we can hopefully re-analyse the flight and actually get the aircraft to fly again digitally.

"That will help investigators a lot to actually determine what was happening in the final moments of the flight."

Local fishermen and members of the local community return to Blacksod, Co Mayo, as the search continues.

Update 10am: The Coast Guard, CHC Ireland and AAIU have held a press briefing at in relation to the Coast Guard helicopter which lost contact early this morning.

Representatives stated that the last communication from Rescue 116 was at 12.45am when they were coming in to refuel at Blacksod after coming from Dublin.

They were due to head out to support Rescue 118 during a medevac operation when they disappeared from screens and communications.

There was no mayday call and the Director of the Coast Guard said they are unsure of what happened.

A search operation is continuing off the west coast after a coastguard helicopter went missing off Blacksod in Co Mayo early today pic.twitter.com/9YX8UcBjMK — RTÉ News (@rtenews) March 14, 2017

A search was then launched for the missing Rescue 116 and the four crew members.

One crew member was recovered who is now in a critical condition. Debris was spotted over a large range.

A large number of rescue vehicles are still on the scene 10km west of Blacksod trying to find the missing crew members.

Coastguard chief Eugene Clonan on vessels and aircraft deployed search and rescue operation off the coast of Mayo #3News pic.twitter.com/l2YQhzrqg6 — Laura Hogan (@LauraHoganTV) March 14, 2017

Update 9.20am: Director of the Coast Guard Eugene Clonan says it is a dark day for the emergency services.

"The Rescue 118 from Sligo, they spotted the wreckage one and a half miles to the south-east of Blackrock Lighthouse. Since then, with daylight and all the efforts on scene, one casualty has been located and is in a critical condition."

He also said that "hope is fading" for the other three crew members.

He said Rescue 116 is a very new one aircraft, a Sikorsky S92, and has had regular maintenance checks.

Update 8am: One person has been recovered as part of the search for a missing Coast Guard helicopter off the west coast.

It is understood the man is in a critical condition and is now on board a lifeboat, being brought ashore.

The coastguard says debris has been found spread over a wide area.

The director of the Irish Coast Guard told Morning Ireland that the last audio message from the crew was: "Shortly landing at Blacksod".

They have also confirmed no distress call was received from Rescue 116 before they lost contact at around 1am.

Greg Dewhurst, senior forecaster with the Met Office, said there were no unusual weather patterns overnight in the Atlantic between the west coast of Ireland and the reported location of the fishing vessel.

He said: "There were no frontal systems bringing heavy rain. Generally it looks like there were some light south-westerly winds. There was quite a bit of cloud around but there were no stormy conditions."

LE Roisín now heading for search area. 1 of missing coastguard crew found. Intense efforts to locate 3 others continues #rtenews #coastguard pic.twitter.com/kepDiofAKR — Joe Mag Raollaigh (@joemagraollaigh) March 14, 2017

Earlier: A search operation is underway for a Coast Guard helicopter missing off the west coast.

The Dublin-based Rescue 116 aircraft had travelled across the country to assist Sligo-based 118 helicopter on a medevac, but contact was lost just before 1am this morning.

Both helicopters refuelled at Blacksod, Co Mayo, prior to transiting to the scene.

In a statement this morning, the Coast Guard said the Sligo-based Rescue 118 helicopter completed an evacuation of a crewman requiring urgent medical attention from a UK-registered fishing vessel 150 nautical miles west of Eagle Island.

"Owing to the distance involved, safety and communication support, known as top cover, was provided by the second Coast Guard helicopter, the Dublin-based Rescue 116," the statement said.

The Shannon based helicopter, two lifeboats and a number of fishing vessels have now joined the search operation, several miles out to sea.

The last known location is thought to be approximately six miles west of Blacksod.

Ger Hegarty is the Divisional controller at the Marine Rescue Coordination Centre in Dublin, he says there was no indication that the helicopter had run into difficulty.

"We lost contact with them about six miles, five or six miles off the coast, and the search is ongoing.

"We lost contact at about 1am this morning with the helicopter. The is four crew on board," he said.

Minister for Transport, Tourism and Sport, Shane Ross TD expressed deep concern at the news this morning that there has been an incident.

“As the search for the Dublin based helicopter is currently underway 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. As we await further information I would like to appeal for space to be given to the relevant professionals involved in the search operation to complete their work. Once again, I send my utmost support to all those affected.”

Rescue 116 was providing topcover for rescue 115 on a medivac off the coast of Erris when contact was lost . pic.twitter.com/gmY3ybbKgG — Fergus Sweeney (@FergusMaSuibhne) March 14, 2017