Mexican officials have released five wolves in the Sierra San Luis of northeastern Sonora, a Mexican environmental group said in a news release.

The wolf release occurred Oct. 11, the group Naturalia said. It gave no specific information about where the release occurred, but that mountain range abuts the New Mexico and Arizona borders with Sonora and ends about 80 miles south of Douglas.

The release came after years of planning by Mexican officials and opposition by U.S. ranchers, who are worried the wolves will cross into the United States and be protected from capture or killing. Environmental groups have said they hope the wolves cross into the U.S. and mix with wolves in Eastern Arizona.