Gruesome video showing men shot dead in the ocean by mystery gunmen highlights problem of murder at sea

A shocking video showing a group of men being gunned down in the ocean has lifted the lid on the high rate of violence on the high seas, and underlined the difficulty of tracking down killers on the water.

The video, which was found last year on a mobile phone in a Fijian taxi, shows four men floating in the water near an overturned boat.

One by one they are shot dead as other ships surround them.

The crew members seen in the video laugh and pose for selfies afterwards.

Despite there likely being dozens of witnesses from at least four ships, the incident was unreported and the identity of the killers remains a mystery.

Fijian authorities have closed their investigation into the crime, saying none of its citizens were involved and it did not take place in Fijian waters, the New York Times has reported.

Taiwanese authorities, who connected a Taiwanese boat to the scene, said they believed the victims had attempted to carry out a failed pirate attack.

However, many maritime security experts have claimed the excuse of "piracy" was used to cover up oceanic violence against mutineers or rival crew.

International Organisation of Masters, Mates and Pilots lawyer Klaus Luhta said it did not matter what the reason behind the massacre was.

"Summary execution, vigilantism, overzealous defense, call it what you will," he said.

"This boils down just the same to a case of murder at sea and a question of why it’s allowed to happen."

Hundreds of mariners are killed at sea every year, but the difficulties of investigating a case in international waters and the reluctance of ship captains and crew to report such incidents mean the culprits are rarely caught.