TUCSON (CN) – Environmental groups on Friday sued the Secretary of the Interior for calling the endangered Mexican gray wolf an “experimental population,” and allowing them to be killed. The Fish and Wildlife Service’s treatment of the Mexican gray wolf as an experimental population limits the number of the wolves in the wild and lets them be killed, the Center for Biological Diversity and Defenders of Wildlife. They sued Interior Secretary Sally Jewell and the U.S. Fish and Wildlife Service, challenging a revised rule placing a cap on the wolf population and allowing the wolves to be captured and killed. The 83 wolves that live in a small area of eastern Arizona and western New Mexico have “not flourished, in significant part because, to date, FWS has imposed numerous restrictions on the Mexican gray wolf reintroduction program that impede efforts to bring this rare subspecies back from the brink of extinction,” the groups say.