It is still unclear why they died: dead dolphins on the beach in Mexico are a mystery.

39 dead dolphins washed up on a beach in the Gulf of California. Laboratory tests and the examination of the animals should now clarify the cause of death, as the network for stranded marine mammals announced. It works in collaboration with the Mexican Prosecutor’s Office for Environmental Protection.

The dead animals were discovered Wednesday on El Califín beach near La Paz in southeastern Mexico’s Baja California peninsula. “The causes of the stranding are still unclear,” said marine biologist Aurora Paniagua of the German Press Agency. The animals showed no injuries caused by humans.

The experts assume that the majority of the dolphins have already been driven ashore dead. Only two or three animals had injuries from rocks and sand caused when the dolphins try to return to the sea, Paniagua said.