Investigators said they are trying to work out how two chimpanzees that viciously attacked a visitor at a California animal sanctuary escaped from their cage.

The chimps chewed off St James Davis' nose and severely mauled his genitals and limbs on Friday before the son-in-law of the sanctuary's owner shot the animals dead.

Davis, 62, and his wife had gone there to visit another chimpanzee that had lived with them for decades.