The Air Accident Investigation Unit has said the extent of the inquiry into the fatal crash of Coast Guard Rescue Helicopter 116 means a final report on the matter will not be completed by the first anniversary of the incident.

Four Coast Guard crew members died when the helicopter crashed into Blackrock Island off the coast of Mayo on 14 March last year.

The bodies of captains Dara Fitzpatrick and Mark Duffy's were recovered following the crash, however, the bodies of winch operator Paul Ormsby and winch man Ciarán Smith have not been located. 

In a statement, the AAIU said that due to the depth and breath of its ongoing investigation, a final report will not be issued before the anniversary of the crash.

Instead, an interim statement will be published in the coming weeks.

This will detail the progress to date and would also outline any safety issues raised.

The inquiry is still gathering evidence and reviewing a considerable volume of material.

A section of the Sikorsky S92 helicopter was removed from the seabed off Blackrock last year.

All parts of the aircraft that have been located in the months since the crash have been brought ashore and logged by gardaí, who then alert the AAIU.

These components are then transferred to the unit's premises at Gormanstown in Co Meath, where a painstaking effort is under way to examine each and every piece of evidence.

The AAIU says it is making "steady progress".

A preliminary report into the accident was released last April. It found that an onboard warning system on the helicopter did not include data relating to Blackrock Island.

In addition, an Enhanced Ground Positioning Warning System did not have the "lighthouse obstacle" included in a database.

The report also found that "the terrain of the island" was not listed on a terrain database.

Mechanical anomalies on the helicopter have been ruled out contributory factors and the ongoing AAIU focus is understood to be on operational aspects of the flight.

These take in several headings but will include human behaviour, navigational controls and procedural issues.