The ongoing battle over a proposal to lift U.S. government protections for the gray wolf (Canis lupus) across the lower 48 states isn’t likely to end quickly. An independent, peer-review panel yesterday gave a thumbs-down to the U.S. Fish and Wildlife Service’s (USFWS’s) plan to delist the wolf. Although not required to reach a consensus, the four researchers on the panel were unanimous in their opinion that the proposal “does not currently represent the ‘best available science.’ ”

“It’s stunning to see a pronouncement like this—that the proposal is not scientifically sound,” says Michael Nelson, an ecologist at Oregon State University, Corvallis, who was not one of the reviewers. Many commentators regard it as a major setback for USFWS, which stumbled last year in a previous attempt to get the science behind its proposal reviewed.

USFWS first released its plan for removing the gray wolf from the endangered species list in June 2013. The plan also called for adding the Mexican gray wolf, a subspecies that inhabits the southwest, to the protected list. At the time, there were approximately 6000 wolves in some Western and upper midwestern states; federal protections were removed from the gray wolf in six of those states in 2011. More than 1 million people have commented on the plan. But regulations also require that the agency invite researchers outside of the agency to assess the proposal’s scientific merit.

At its core, the USFWS proposal relies on a monograph written by its own scientists. They asserted that a different (and controversial) species, the eastern wolf (Canis lupus lycaon) and not the gray wolf, had inhabited the Midwest and Northeast. If correct, then the agency would not need to restore the gray wolf population in 22 eastern states, where gray wolves are no longer found.

But the four reviewers, which included specialists on wolf genetics, disagreed with USFWS’s idea of a separate eastern wolf, stating that the notion “was not universally accepted and that the issue was ‘not settled’ ”—an opinion shared by other researchers. “The designation of an ‘eastern wolf’ is not well-supported,” says Carlos Carroll, a conservation biologist at the Klamath Center for Conservation Research in Orleans, California, who was not a member of the review panel.

Overall, the agency’s “driving goal seemed to be to identify the eastern wolf as a separate species, and to use that taxonomic revision to delist the gray wolf,” says Robert Wayne, a conservationist geneticist at the University of California (UC), Los Angeles, and one of the reviewers. If that were to happen, he says, it would be the first time that a species was removed from the federal endangered species list via taxonomy. “It should happen when a species is fully recovered,” Wayne says, “and the gray wolf is not. It’s not in any of those 22 eastern states—that’s why it’s endangered there.”

The panel’s statements will make it difficult, outside observers say, for USFWS to move forward with its proposal. The Endangered Species Act requires that decisions to remove a species from federal protection be based on the “best available science.” And because the reviewers have concluded this is not the case, “you’ve got to think that the [service] must go back to the drawing board,” says Andrew Wetzler, director of land and wildlife programs for the Natural Resources Defense Council in Chicago, Illinois, an organization that advocates for continued federal protections for the wolf.

Gray wolves were exterminated across most of the lower 48 states in the last century. They were placed on the endangered species list in 1975, and successfully reintroduced into Yellowstone National Park and Idaho in 1995. Gray wolves also made a comeback in the Great Lakes region, where they now can be legally hunted. Idaho, Wyoming, and Montana also have wolf hunting and trapping seasons. Smaller gray wolf populations that aren’t legally hunted are found in Washington and Oregon.

The agency’s reaction to the peer-review comments has been somewhat muted. In a press statement, it thanked the National Center for Ecological Analysis and Synthesis at UC Santa Barbara for conducting the review. USFWS Director Dan Ashe noted that “[p]eer review is an important step in our efforts to assure that the final decision on our proposal to delist the wolf is based on the best available scientific and technical information,” and that the panel’s comments will be incorporated in the ongoing process of reaching a decision on the fate of the gray wolves.

The peer-review report is now available online. USFWS will reopen the public comment period on its delisting proporal on 10 February, and will accept comments through 27 March.