Crimes like this don't often happen on land. A 10-minute slow-motion slaughter captured by a cellphone camera showed a group of unarmed men at sea, flailing in the water, shot and killed one by one, after which the culprits posed for celebratory selfies. The only thing more shocking than the footage was the government inaction that followed.

The case shows the challenge of prosecuting crimes on the high seas and the reason violence offshore often occurs with impunity. There were at least four ships at the scene that day, but no law required any of the dozens of witnesses to report the killings — and no one did. Authorities learned of the killings only when the video turned up on a cellphone left in a taxi in Fiji in 2014.

It’s still unclear who the victims were or why they were shot. An unknown number of similar killings take place each year — deckhands on the ship from which the video was shot later said they’d witnessed a similar slaughter a week before.