Royal Navy divers are to be brought in to explore a mystery wreck in the River Foyle which some believe could be a war-time submarine.
Experts met in Londonderry on Tuesday as speculation grew about what the wreck could be.
Meanwhile, sonar images have been published appearing to show a submarine shaped object on the river bed.
The images were taken by Mallow Search and Rescue. The wreck was found during the search for a missing man.
John Wolf, of Mallow Search and Rescue, told RTE's Morning Ireland: "We were scanning the river bed and an image came up on the scanner and it looks like a submarine.
"We're not saying that it is a submarine, but it looks like the real thing all right."
At the end of the Second World War, German submarines surrendered in the Foyle from where they were taken out to sea and destroyed or sunk.
There is speculation that the vessel could be a German mini or midget submarine.
George Malcolmson, who works at the Royal Navy Submarine Museum in Gosport, has viewed the sonar image and said he did not believe the craft was British.
However, he said a picture of surrendered German boats could be a strong clue on what the craft is.
"My attention was drawn to a photograph showing all the German submarines lined up against the jetty and the fact that there was a very small unidentified submarine amongst those," he told Morning Ireland.
"We're hoping this might help officials positively identify the wreck."
Northern Ireland Environment Minister Alex Attwood said its identity should be known within two weeks.
"Given the risks around this operation, given that - for all I know - there could be munitions on board, we have to employ a specialist team with specialist knowledge and specialist skills," Mr Attwood said.
"So far, all we have are the sonar images of what lies on the bed. Nothing can be ruled in or ruled out.
"It could be a yacht, it could be a mini-sub, it could be some piece of metal."