The relatives of 20 men who disappeared while visiting Acapulco on 30 September will help determine if 18 bodies found in a mass grave are those of the abducted tourists.
Police were tipped about the fosa clandestina after a video appeared on YouTube where two men admitted killing and burying the men from Michoancan.
One relative said : "We have been asked to identify whether the bodies are our relatives. We have faith that this is not true and that they are still alive."