Updated at 12:15 a.m., Dec. 15, 2014
Zoo officials said Angalifu, thought to 44, may have died from old age Sunday at the Safari Park, leaving just five members of the species worldwide.
The zoo’s Christina Simmons said results from laboratory tests might “take some time.”
Another spokeswoman for the park, Darla Davis, said the prognosis for the species appears “very bleak.”
Zookeepers unsuccessfully tried to get the Safari Park’s northern white rhinos to mate.
The only members of the species left are an elderly female at the Safari Park, three in Africa and one at a zoo in the Czech Republic.
The northern white rhino has been nearly wiped out by hunting and poaching for their horns, and by wars in Africa, according to the World Wildlife Foundation.