A section of the missing Coast Guard helicopter R116 has been found by Air Accident Investigation Unit inspectors on Blackrock, some 10 kilometres off the Mayo coast.
The discovery was made close to Blackrock lighthouse this evening, when personnel from the AAIU were brought to the site by an Air Corps helicopter.
The finding confirms that the helicopter encountered difficulties around the area where the last communication with it was registered.
Investigators spent around 90 minutes on the small island, which houses the most westerly lighthouse off Co Mayo.
Chief Inspector with the Air Accident Investigation Unit Jurgen Whyte has said that no impact marks of significance were found on the lighthouse itself.
Poor weather conditions hampered efforts to locate the black box flight recorder from the helicopter today.
Search teams, close to the site where a signal from the recorder was detected, spent the day carrying out surface monitoring of the waters around Blackrock lighthouse. The signal was detected in waters around 60 metres from the island on Wednesday evening.
But heavy sea swells impeded efforts to carry out further underwater investigations.
The search operation will resume in the morning.
The discovery of the black box was described as a significant step in the effort to locate the wreckage of Rescue 116. It lost contact with Malin Head Coast Guard base in the early hours of Tuesday morning and crashed soon afterwards.
The main focus of the search operation is to locate three missing crew: Captain Mark Duffy, winch operator Paul Ormsby and winchman Ciarán Smith.
A fourth crew member, Captain Dara Fitzpatrick, was recovered from the water off the Mayo coast on Tuesday morning but pronounced dead a short time later.
Two local vessels approached the main search site this morning, in an effort to home in on the black box recorder signal.
The boats were accompanied by two patrol vessels but conditions as they approached Blackrock were not good and for safety reasons the boats returned to Blacksod and remained there all afternoon.
An extensive trawl around the shores of Blacksod Bay was under way all day and air searches at sea also proceeded.
The agencies involved in the operation are reviewing conditions on an almost hourly basis and said any opportunity to progress the search would be seized upon.
Air Accident Investigation Unit inspectors will not speculate on possible causes, saying their task is to establish the facts.
The entire investigation could take months to complete but priority is being given to locating the missing crew.
Local fishermen have said the currents of water around Blackrock are difficult to negotiate.
The outlook is for rain and poor visibility tomorrow.
Given the conditions at sea yesterday, specialist equipment had to be transferred on to local boats the Ros Anne and Bunacurry Lass, which were able to navigate closer to Blackrock Lighthouse than bigger vessels involved in the search.
The Commissioners of Irish Lights has sent the Granuaile to the area where it will be on standby to assist the recovery operation. The vessel is fitted with a 20 tonne crane which will be used to lift the helicopter wreckage if it is located.
Mr Whyte said the AAIU believes the black box will be located with the wreckage of the helicopter and there is a strong likelihood that the bodies of the missing crewmen will also be in that area as well.
Mr Whyte said the area they will be searching is very challenging.
"The general area that we're searching has areas of rock which protrude out of the water and, on either side, drops down to 40 or 50 metres in depth.
"The reason that we're focusing on the recorder is, we believe the recorder will be either in the wreckage or in close proximity to the wreckage.
"We also believe the likelihood is that the three missing persons will be in that area as well."