Duncan testified before a grand jury in Houston last week about the hunting trip in which he killed a moose and a sheep while flying with a Russian guide. He said he wasn't aware hunting from the air was illegal in Russia, as it is in the U.S.
"The Humane Society of the United States urges prosecution to the full extent of the law and we thank the Department of Justice for giving this case the attention it deserves," the group said in a statement today.
Duncan's attorney, Rusty Hardin, said he "respects everyone's right to have an opinion" but would not comment further on the group's statement.
Hardin said last week that the government might prosecute Duncan under the Lacey Act, a law designed to prevent the interstate and international trafficking of rare plants and animals, although the animals he shot were not endangered.