Marseille, France -- Two divers in France say they discovered three dead dolphins in the Mediterranean that had been tied by their tails to a concrete block.

The dolphins, believed to be a mother and her two young, were found off the coast of Marseille,The Local reported Friday.

"We were at a depth of about 55 meters [182 feet] when we noticed an unusually shaped light-colored mass just underneath us," said Fabienne Henri, who was diving with another woman, Jacqueline Dozin.

"As we got closer, we could make out the shapes of three dead dolphins lying on the sand," she said.

"I'm not an expert, but they had not been there longer than 10 minutes," Henri said. "To me it seems obvious they were thrown from the surface. Why there? I have no idea."

The Marseille prosecutor's office said it had begun an inquiry.

"These mammals are protected and killing them is a criminal offense for which the guilty could go to prison," a representative of the maritime police said.