WASHINGTON  The Justice Department has told Congress that American 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 Central Intelligence Agency. 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 C.I.A. 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.