The Mexican gray wolf population in the United States dropped 12 percent in 2015, with the U.S. Fish and Wildlife Service blaming the decline on an increase in adult wolf deaths and a decrease in the survival rate of pups.

According to federal wildlife officials’ latest survey, 97 of the endangered wolves were found in Arizona and New Mexico last year, compared with 110 counted in 2014. The decline comes following five consecutive years of annual population increases in wolf numbers.

“These latest population numbers demonstrate we still have more work to do in stabilizing this experimental population and maximizing its anticipated contribution to Mexican wolf recovery,” Benjamin Tuggle, the agency’s Southwest director, said in a statement.

Sherry Barrett, the wildlife service’s Mexican wolf recovery coordinator, said the next step is to determine if last year’s decline was just an anomaly or if wolves in both states are facing new challenges to survival.

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Conservation groups say the endangered animals are stalling on the road to recovery owing to illegal wolf killings at the hands of cattle and livestock owners.

Thirteen adult wolves were found dead in 2015, up from 11 in 2014, and 11 wolves were considered “fate unknown,” compared with just three in 2014.

Michael Robinson of the Center for Biological Diversity said the telemetry receivers loaned out by federal and state officials to livestock owners need to be banned. The devices allow ranchers to track radio-collared wolves if they get near their property. Robinson said the receivers have been loaned out for years, despite high rates of unsolved illegal shootings as well as the disappearances of many radio-collared wolves under suspicious circumstances.

“It’s unconscionable to give high-tech tools that strip the wolves of their ability to stay hidden to the very people who have expressed their hatred for wolves and oppose these animals’ recovery,” Robinson said.

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The U.S. wolves are part of an experimental population released back into the wild in 1998 after the species was nearly exterminated in the 1970s. Their reintroduction has been controversial, with conservationists praising the return of a species to its historical range and ranchers decrying adding a potential threat to their livestock.

With the population hovering around 100 in two states, fewer surviving gray wolf pups could mean more declines in future surveys.

Fifty-five percent of wolves born in 2015 survived through December—compared with the 84 percent survival rate in 2014.

“Of the 21 wolf packs on the ground today, 10 successfully reared a litter through the end of this year,” Jim deVos, assistant director of wildlife management for the Arizona Game and Fish Department, said in a statement. “Wildlife populations vary on an annual basis, so the decline in the number of Mexican wolves counted this year is not out of character.”

The Center for Biological Diversity says the issue is inbreeding—more wolves from captive-breeding facilities need to be released into the wild to increase genetic diversity.

“Endless delay in releasing wolves into the wild to address the genetic crisis results in inbred wolf pups that cannot survive,” Robinson said. “Our government must stop placating livestock interests and start prioritizing saving the Mexican wolf before it’s too late.”

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Original article from TakePart