As many as 21 dolphins died after washing up on a shore following an attack by another species of dolphin, the Federal Office of Environmental Protection, or Profepa, in Mexico said on Tuesday.

Profepa said that environmental activists launched an operation to save the lives of 54 short-beaked dolphins who had washed up on a rocky beach in Bahia de la Paz, in Mexico’s Baja California peninsula. They managed to save 33 of the mammals by putting them back in the water, but 21 others died.

Rescuers found bite marks on the dead dolphins’ bodies, which indicated that they had been attacked by bottle-nosed dolphins, Profepa said. However, the office added that staff from the veterinary medicine unit of the Autonomous University of Baja California Sur would conduct an autopsy on the animals’ bodies to precisely determine their cause of death.

The federal office said that the dolphins’ carcasses would be buried using heavy machinery, as per the protocol for responding to strandings of marine mammals.

Some species of dolphins are known to kill other dolphins.