By Laura Zuckerman

July 10 (Reuters) - Conservationists on Friday protested a U.S. decision not to prosecute a Utah hunter who mistakenly shot and killed a gray wolf that had gained an international following for being the first of its kind seen at the Grand Canyon in 70 years.

The wandering wolf, a radio-collared female from Wyoming nicknamed "Echo," was celebrated last fall after being spotted in and around the national park in Arizona before a Utah coyote hunter reported in December that he had shot and killed the wolf after mistaking it for a coyote.

It is illegal to kill wolves without a special permit in the Lower 48 states where most are protected under the U.S. Endangered Species Act. Wolves in just two states, Idaho and Montana, are not on the federal list of endangered and threatened species and can be legally hunted.

A probe by the U.S. Fish and Wildlife Service, the Utah Division of Wildlife Resources and the U.S. Justice Department found that the hunter did not violate federal or state laws by mistakenly killing the wolf.

The Endangered Species Act makes it a crime for anyone to knowingly violate the law, including provisions that prohibit harassment, harm or death of protected animals. But officials said in a statement that the shooting of Echo resulted from misidentification, and was not intentional.

Michael Robinson, conservation advocate with the Center for Biological Diversity, criticized the decision and the policy of requiring proof a person knew the animal harmed or killed was protected.

"That's a standard that is almost impossible to meet and it leaves a lot of endangered animals very vulnerable," he said.

Authorities have not released the name of the Utah coyote hunter. Hunting regulations in Utah and elsewhere require sportsmen to properly identify targets before seeking to harvest them.

State wildlife officials said on Friday they were tweaking a hunting program to help sportsmen distinguish between wolves and coyotes, which may be shot on sight in Utah and much of the nation.

Echo was one of two protected gray wolves in the southern Rockies that have been shot and killed in recent months by coyote hunters.

The Fish and Wildlife Service has said testing confirmed an animal killed in Colorado in April was the first confirmed gray wolf in the state since 2009. That hunter also said he thought the wolf was a coyote and immediately informed wildlife officials of his mistake, authorities said. (Reporting by Laura Zuckerman in Salmon, Idaho; Editing By Cynthia Johnston and Sandra Maler)