Probe into whether helicopter attempted to land on Black Rock
Officials are probing whether the Rescue 116 helicopter tried to make an emergency landing at Black Rock lighthouse before the fatal crash.
It is believed that Captain Dara Fitzpatrick made a desperate attempt to save her helicopter and crew when it encountered "catastrophic" difficulties.
The search continues for the Coast Guard's Sikorsky S92 aircraft and the bodies of three men who have not been found since the helicopter disappeared without warning off Co Mayo early on Tuesday.
Debris has been located on Black Rock, a small island with a lighthouse and helipad 13km west of Blacksod on the Mayo coastline where the helicopter was due to refuel.
Investigators are now examining the theory that the aircraft suffered a sudden and serious mechanical failure which forced an emergency landing.
It comes as officials hope a "three-hour window" between low tides will be sufficient to find the submerged aircraft, amid fierce weather conditions.
They believe they have identified from preliminary sonar scans and beacon triangulation data the main wreckage of the downed Irish Coast Guard helicopter on the seabed.
Naval Service and salvage experts are now "very hopeful" that the bodies of the three missing Rescue 116 crewmen will be found trapped within the wrecked fuselage of the helicopter.
The wreckage lies at a depth of 40 metres, some 60 metres off Black Rock Island and lighthouse.
Sources have said that the beacon of the lighthouse located close to where the helicopter wreckage was found had been functioning throughout Monday night and Tuesday morning.
A source said investigators airlifted onto Black Rock would be meticulous in recording where the debris was located.
The pattern of debris on the ground could determine the direction of impact and provide clues to what went wrong.
"A number of potential scenarios are being investigated," a source said. "One is that the aircraft began to develop catastrophic mechanical difficulty as they were in the area of Black Rock.
"Were they trying to make an emergency landing on Black Rock before a collision?"
There was no mayday or radio call before the tragic accident just before 1am on Tuesday, leading investigators to believe "it all happened extremely quickly".
Rescuers are also mystified as to how alert devices on board the helicopter were not triggered after the crash.
Debris from the helicopter has been found on Black Rock Island and as far south as Achill and Annagh Islands.
All wreckage is being carefully gathered by Air Accident Investigation Unit (AAIU) officials and will be painstakingly re-assembled in a bid to determine what happened to Rescue 116.
Captain Fitzpatrick was recovered from the sea on Tuesday morning but was pronounced dead on arrival at hospital.
Her funeral takes place in Glencullen this morning.
The highly experienced and ground-breaking pilot, well known to Irish television audiences for her role in a fly-on-the-wall documentary about the Coast Guard, was the mother of a three-year-old son.
Her three crewmen - Ciarán Smith, Mark Duffy and Paul Ormsby - are still missing.
Derek Geoghegan of the Irish Coast Guard said that a small window of three hours had been identified on Monday.