(CNN) A rhino is dead after poachers sawed off one of its horns at a zoo in the suburbs of Paris.

This is believed to be the first time a rhino has been poached while inside a zoo, the zoo said.

The poachers broke into the Thoiry Zoo on Monday night, forcing an outer gate and several doors open, before killing the rhino, the zoo said.

"They shot dead Vince, a 4-year-old male, to cut one of its horns, probably with a chainsaw," it said. "Its second horn was partially cut, which left the impression that the criminals were disturbed or that their equipment was defective."

Two other rhinos in the enclosure were unharmed.

"The whole staff is extremely shocked," the zoo said.

Vince was a part of the Southern white rhino subspecies, and because they are considered extremely endangered, reproduction and conservation in zoos is essential.

All five rhino species are on the endangered list, and are often the victims of poaching because some people believe their horns have medicinal properties. In 2015, 1 kilogram of rhino horns was sold on the black market for 51,000 Euros.

Illegal wildlife trading is one of the largest trades in the world, after drugs, counterfeiting, and human trafficking.