Richard Emslie of the World Conservation Union's Species Survival Commission said that experts had searched 1,200 miles of habitat in northern Cameroon, but failed to find any sign of the West African black rhino.
Emslie, a rhino expert based in KwaZulu-Natal Province, South Africa, said: "They looked for spoor (tracks or droppings), they looked for the rhino's characteristic way of feeding, which has an effect like a pruning shear."
Although not discovering any sign of black rhinos, the experts did come across evidence of poaching.
Poachers have long hunted the animals for their horns, which are used in traditional Asian medicine to fight malaria, epilepsy and other ailments. In Yemen, the horns are in demand for use as carved handles on traditional daggers.
Conservationists estimated that there were more than 100,000 West African black rhinos in 1960. That figure dropped to an estimated 14,000 by 1980 and the animals may now be extinct.