An endangered squirrel species found only in the Pinaleño Mountains of Southeastern Arizona was heavily impacted by a forest fire this summer, federal wildlife officials say.

According to a new survey, Mount Graham red squirrels, found only on their namesake peak and other mountains in the Pinaleños, now number only 35 in the wild, the Arizona Daily Star reported last week. That's a dramatic decrease from the more than 250 of the squirrels estimated in last year's survey, the U.S. Fish and Wildlife Service said.

The decrease is due to the Frye Fire, a lightning-caused blaze that burned 48,000 acres in the Pinaleños this summer, a Fish and Wildlife Service official said. Steve Spangle, Arizona field supervisor for the agency, added that he's "not optimistic" about the species' survival.

“We’re very worried about a critter going extinct under our watch,” Spangle told the Daily Star. “It doesn’t mean we won’t do what we can."

The annual squirrel census found evidence of fire activity in 95 percent of surveyed areas, and 80 percent of those areas showed at least some habitat loss.

Mount Graham red squirrels, one of more than 20 known subspecies of red squirrels in North America, were listed as endangered in 1987. Besides the Pinaleños, the only other known living specimens are five squirrels in captivity at the Phoenix Zoo, the Daily Star reported.