ANCHORAGE, Alaska - A federal court judge has given the Interior Department a Dec. 23 deadline to explain why polar bears were listed in 2008 as "threatened" instead of the more-protective "endangered."

The written order issued Thursday by Judge Emmet Sullivan of the U.S. District Court in Washington, D.C., follows an October hearing.

Sullivan writes that the U.S. Fish and Wildlife Service erroneously concludes that a species must be in imminent danger of extinction to be declared endangered.

Sullivan says that runs counter to the plain meaning of the Endangered Species Act.

Former Interior Secretary Dirk Kempthorne in 2008 declared polar bears were threatened, or on the way to extinction, because of the rapid disappearance of the Arctic sea ice.

The state of Alaska argues that polar bears should not even be listed as threatened.