WASHINGTON  A newly disclosed Justice Department legal memorandum, written in March 2003 and authorizing the military’s use of extremely harsh interrogation techniques, offers what could be a revealing clue in an unsolved mystery: What responsibility did top Pentagon and Bush administration officials have for abuses committed by American troops at the Abu Ghraib prison in Iraq and in Afghanistan; Guantánamo Bay, Cuba; and elsewhere?

Some legal experts and advocates said Wednesday that the document, written the month that the United States invaded Iraq, adds to evidence that the abuse of prisoners in military custody may have involved signals from higher officials and not just irresponsible actions by low-level personnel.

The opinion was written by John C. Yoo of the Office of Legal Counsel, the executive branch’s highest authority on the interpretation of the law. It told the Pentagon’s senior leadership that inflicting pain would not be considered torture unless it caused “death, organ failure or permanent damage,” and it is the most fully developed legal justification that has yet come to light for inflicting physical and mental pressure on suspects.

While resembling an August 2002 memorandum drafted largely by Mr. Yoo, the March 2003 opinion went further, arguing more explicitly that the president’s war powers could trump the law against torture, which it said could not constitutionally be enforced if it interfered with the commander in chief’s orders.