WASHINGTON - The Justice Department has told Congress that US intelligence operatives attempting to thwart terrorist attacks can legally use interrogation methods that might otherwise be prohibited under international law.

The legal interpretation, outlined in recent letters, sheds new light on the still-secret rules for interrogations by the CIA. It shows that the administration is arguing that the boundaries for interrogations should be subject to some latitude, even under an executive order issued last summer that President Bush said meant that the CIA would comply with international strictures against harsh treatment of detainees.

While the Geneva Conventions prohibit "outrages upon personal dignity," a letter sent by the Justice Department to Congress on March 5 makes clear that the administration has not drawn a precise line in deciding which interrogation methods would violate that standard, and is reserving the right to make case-by-case judgments.

"The fact that an act is undertaken to prevent a threatened terrorist attack, rather than for the purpose of humiliation or abuse, would be relevant to a reasonable observer in measuring the outrageousness of the act," said Brian A. Benczkowski, a deputy assistant attorney general, in the letter, which had not previously been made public.

Bush issued the executive order last summer to comply with restrictions imposed by the Supreme Court and Congress. The order spelled out new standards for interrogation techniques, requiring that they comply with international standards for humane treatment, but it did not identify any approved techniques.

It has been clear that the order preserved at least some of the latitude that Bush has permitted the CIA in using harsher interrogation techniques than those permitted by the military or other agencies. But the new documents provide more details about how the administration intends to determine whether a specific technique would be legal, depending on the circumstances involved.

The letters from the Justice Department to Congress were provided by the staff of Senator Ron Wyden, Democrat of Oregon, who is a member of the Intelligence Committee and had sought more information from the department.

Some legal specialists critical of the Justice Department interpretation said the department seemed to be arguing that the prospect of thwarting a terror attack could be used to justify interrogation methods that would otherwise be illegal.

"What they are saying is that if my intent is to defend the United States rather than to humiliate you, then I have not committed an offense," said Scott L. Silliman, who teaches national security law at Duke University.

But a senior Justice Department official strongly challenged this interpretation on Friday, saying that the purpose of the interrogation would be just one among many factors weighed in determining whether a specific procedure could be used.

"I certainly don't want to suggest that if there's a good purpose you can head off and humiliate and degrade someone," said the official, speaking on condition of anonymity because he was describing some legal judgments that remain classified.

"The fact that you are doing something for a legitimate security purpose would be relevant, but there are things that a reasonable observer would deem to be outrageous," he said.

At the same time, the official said, "there are certainly things that can be insulting that would not raise to the level of an outrage on personal dignity."

The humiliating and degrading treatment of prisoners is prohibited by Common Article 3 of the Geneva Conventions.

The legal reasoning included in the latest Justice Department letters is less expansive than what department lawyers offered as recently as 2005 in defending the use of aggressive techniques. But they show that the Bush administration lawyers are citing the sometimes vague language of the Geneva Conventions to support the idea that interrogators should not be bound by ironclad rules.

In one letter written Sept. 27, 2007, Benczkowski said that "to rise to the level of an outrage" and thus be prohibited under the Geneva Conventions, conduct "must be so deplorable that the reasonable observer would recognize it as something that should be universally condemned."

Wyden said he was concerned that, under the new rules, the Bush administration had put Geneva Convention restrictions on a "sliding scale."

If the United States used subjective standards in applying its interrogation rules, he said, then potential enemies might adopt different standards of treatment for American detainees based on an officer's rank or other factors.

"The cumulative effect in my interpretation is to put American troops at risk," Wyden said.

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