Air accident investigators have warned they will not complete the report into the crash of an Irish Coast Guard helicopter by the first anniversary.
Rescue 116 crashed off Co Mayo at 12.46am on March 14 last year with four crew on board after it struck Blackrock Island 12 miles off the coast.
Captain Dara Fitzpatrick, the commander of the flight, was pulled from the sea in the hours after the crash and never regained consciousness, while the body of Captain Mark Duffy, the co-pilot, was taken from the cockpit 12 days later by Navy divers.
The bodies of winchmen Paul Ormsby and Ciaran Smith were never recovered despite weeks of intensive searches of the seabed, surface and shore.
The Air Accident Investigation Unit (AAIU) said it was still in the process of gathering factual and background information and is making steady progress.
But investigators said they would not complete their work within 12 months from the date of the accident as preferred under international convention and law.
An interim statement detailing progress and any safety issues is being planned, but no date for publication was confirmed.
"The AAIU again extends its condolences to the families and friends of those who lost their lives in this accident," investigators said.
"The AAIU wishes to advise that due to the depth and breadth of this investigation, it will not be possible to issue a final report within 12 months of the date of the accident and therefore an interim statement will be published.
"The investigation is endeavouring to issue this interim statement before the anniversary; however, it is not possible to say at this time when the interim statement will be published."
The AAIU previously confirmed that it has established that the Sikorksy S-92's internal warning system did not include the 80m (262ft) high rocky outcrop of Blackrock and its working lighthouse in its database.
The search and rescue helicopter crashed after hitting the island while coming back from supporting a mission for an injured trawlerman out in the Atlantic.
The AAIU preliminary report stated that Rescue 116's Honeywell Enhanced Ground Proximity Warning System (EGPWS), which alerts pilots both visually and aurally to imminent danger from terrain and obstacles, does not include either the terrain of Blackrock or the lighthouse.
CHC Ireland, which operates the Irish Coast Guard search and rescue helicopters, was asked to review and re-evaluate all route guides used in the aircraft.