ACCIDENT investigators are to raise the wreck of a fishing vessel which sank off the Outer Hebrides over three weeks' ago as they try to discover what caused its sinking.

The bodies of two crew members Chris Morrison, 27, and Martin Johnstone, 29, were found, following the tragedy off Mingulay on April 9. The skipper Paul Alliston, 42, from Lewis is still missing and presumed dead.

The Marine Accidents Investigation Branch (MAIB) have now said that they would be salvaging the Louisa as part of their investigation into what happened.

One of the four crew Lachlan Armstrong, 27, survived the sinking of the 15m (49ft) crabber on April 9 as it lay at anchor in calm seas.

The boat, which was used for crab fishing, was understood to have taken on water, forcing the crew to abandon ship.

Investigators want to know why water flooded into the boat and whether automatic alarms that should sound when a leak is detected were activated.

They also want to discover why the Louisa's liferaft failed to release and then did not inflate.

It is understood the crew tried to push buoys under the stricken raft to keep it on the surface.

But the rubber canopy started to sink, leaving the four men left clinging to the side with only their lifejackets keeping them afloat.

A spokesman for the MAIB said: "We have made a decision to raise the wreck and will do so when there is a suitable weather window."

The boat lies at the comparatively shallow depth of 10m (33ft), with its mast visible at low water.

The MAIB spokesman added: "The aim of our investigation is to establish why the fishing vessel Louisa sank and why lives were lost. A full and detailed report will be published at the end of our investigation.

"If we identify any safety issues during the investigation that require immediate action, the Chief Inspector of Marine Accidents will issue a safety bulletin or make recommendations as appropriate."

Mr Armstrong from Stornoway who survived after swimming ashore was eventually found by Barra lifeboat crew.

The bodies of Mr Johnstone of Halkirk, Caithness, and Mr Morrison, 27, of Leverburgh, Harris, were recovered from the water.

The crab boat is jointly owned by brothers Duncan and Murdo Kennedy from Lewis who in the aftermath of the accident said they were “devastated” by the tragedy.

They added: “We are engaging fully with representatives from MAIB, officers from Police Scotland and the vessels insurers, as they continue their comprehensive investigation into events which occurred into and following the sinking of the Louisa.”

Mr Armstrong said afterwards that the liferaft had failed to inflate and his three colleagues had done all they could to make it float.

"The water was so cold and we were all immersed in it," he said.

“Being that cold you know the only way you can survive is to get out of the water."

He said the combined weight of all the men on the raft was not helping the situation and he "made the choice to try to go for land".

"I didn’t know if I’d make it," he said.

“I thought leaving the others would mean they’d have more of a chance to get on the raft.

“You were so cold you couldn’t even hold on to it. I have no idea how I found the strength to keep going. When I did finally reach the rocks I was convinced the rest of them would be OK. To hear that they were all gone - it still hasn’t sunk in.”

It was later reported that rescuers found the body of Mr Alliston in the water during their search, but he vanished under the surface as they tried to reach him.