PORTLAND — A rare black rhinoceros died while being transported to Portland from Kansas City for breeding purposes, the Oregon Zoo said Tuesday.

The rhino became agitated early Monday about 20 miles outside Phoenix, causing the truck to sway. Zoo staff immediately stopped the truck to check on her before heading to the city’s zoo to seek care.

The Phoenix Zoo had a crane delivered and the rhino was unloaded from the truck into an elephant barn. The rhino died Monday night from unknown causes.

“It just doesn’t always go the way you hope it does,” said Oregon Zoo veterinarian Dr. Lisa Harrenstien, who flew to Arizona on Monday to help care for the animal. “It can take a turn for the worse and you do what you can to get back on track.”

The Phoenix Zoo is performing a necropsy to determine the cause of death, said Oregon Zoo spokesman Bill LaMarche. Preliminary results should be available within three weeks.

There are an estimated 3,500 black rhinos left in the world and 67 are held in captivity, LaMarche said. “Poachers hunt them mercilessly for their horn,” he said. “The species is really on the brink.”

LaMarche said staffers transporting the rhinoceros from Kansas City, where it was born in 2000, took a detour through Arizona because of snow advisories in Wyoming.

The Species Survival Plan, which is run by the Association of Zoos and Aquariums, had called for the 8-year-old female named Kipenzi, which means “precious one” in Swahili, to breed with the Oregon Zoo’s 21-year-old male.

LaMarche said another female will likely be sent to the Oregon Zoo through the plan, which aims to sustain a genetically diverse population among animals in North American zoos.

Black rhinos are native to Eastern and Central Africa.