Three dolphins have been found dead â€“ tied to a concrete block by their tails.
The grim discovery was made by two divers off the coast of Marseille, France.
The dolphins were believed to be a mother and her two young.
One of the divers, Fabienne Henri said: "We were at a depth of about 55 metres when we noticed an unusually shaped light-coloured mass just underneath us.
"As we got closer, we could make out the shapes of three dead dolphins lying on the sand."
"I'm not an expert but they had not been there longer than ten minutes. To me it seems obvious they were thrown from the surface. Why there? I have no idea."
The Marseille prosecutor has opened an inquiry. An autopsy will be carried out to establish the cause of the dolphins' deaths.
"These mammals are protected and killing them is a criminal offence for which the guilty could go to prison," said a representative for the maritime police.
Earlier in the year the bodies of two rare Australian snubnose dolphins were found tied to a block of concrete at Toolakea Beach north of Townsville, Australia. It was thought they had probably been caught accidentally in fishing nets, but rather than reporting the incident there had been a clear attempt to conceal the bodies.
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