Justice Department spokeswoman Patti Pontello in Sacramento said the 1998 policy accounted for the lesser charge in the condor case. Lewis’ attorney declined to comment.

Federal wildlife officials, who have tried to persuade the Bush administration to change the policy, say it has blocked them from bringing charges under the endangered species law in dozens of cases around the country.

Before the policy went into effect, prosecutors needed only to show that a suspect knowingly aimed at and shot an animal and that the dead animal was a member of an endangered species.

But now, “we have to show a mental state, which for federal agents is very difficult. It’s hard for anyone to read someone’s mind,” said Neill Hartman, acting agent-in-charge for eight Western states for the U.S. Fish and Wildlife Service.

“If you don’t prosecute, who’s to stop someone from going out and taking a bear just because they want a grizzly in their trophy room?”

The policy stemmed from a Montana case in which a man named Chad McKittrick was given a six-month sentence for killing a wolf in 1995 near Yellowstone National Park. McKittrick argued that he could not be convicted because he thought he was shooting a wild dog.

The judge in his case sided with prosecutors and told jurors they could convict McKittrick if they found that he had “knowingly engaged in a taking of an animal” and that, in fact, “the animal was a threatened species of wildlife, namely a gray wolf.”

A federal appeals court upheld the conviction, saying that the jury instructions were in line with those used in other endangered species cases.

But when the case reached the U.S. Supreme Court, Justice Department lawyers reexamined the issue. They decided that the jury instructions prosecutors had argued for were wrong.

Under previous Supreme Court rulings, they decided, a person could not be convicted of “knowingly” killing an endangered animal unless prosecutors could prove that the person actually knew what kind of animal he or she had shot.

That decision brought the Endangered Species Act into line with many other federal laws, said Lois Schiffer, assistant attorney general for environment and natural resources in the Clinton administration.

But many federal wildlife officials said the policy has given those who kill endangered animals an easy way to escape prosecution, at least under the Endangered Species Act. Other federal and state laws can sometimes be used, as in the condor case, but those laws generally carry lesser penalties.

In Montana and Wyoming — home to most grizzly bears in the Lower 48 states — the policy already has effectively halted federal criminal prosecutions in shooting of grizzlies and wolves, Hartman said.

Even under the old policy, prosecutors faced difficulties in wolf and grizzly shootings because defendants often argued they had shot in self-defense. But the McKittrick policy has blocked Endangered Species Act prosecutions even in cases that did not involve self-defense, officials said.

In Wyoming, for example, the policy has blocked federal criminal charges in the killing of five grizzly bears and two wolves. In Colorado, prosecutors could not charge a man who shot a lynx; he said he thought he was aiming at a bobcat.

Former Fish and Wildlife Director Jamie Rappaport Clark appealed more than two years ago to then-Interior Department Solicitor John Leshy in hopes of changing the policy. The attempt failed.

“As feared, this … requirement has precluded criminal prosecution in direct taking [shooting] cases around the country,” Clark wrote.