In a court filing Monday, government attorneys say "a recent unexpected delay" is indefinitely holding up action on the predators. No further explanation was offered.
Gray wolves are under protection as an endangered species and have recovered dramatically from widespread extermination in recent decades.
More than 6,000 of the animals now roam the continental U.S. Most live in the Northern Rockies and western Great Lakes, where protections already have been lifted.
A draft proposal to lift protections elsewhere drew strong objections when it was revealed last month.
Wildlife advocates and some members of Congress argue that the wolf's recovery is incomplete because the animal occupies just a fraction of its historical range.
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