Still, Jennifer Daskal, senior counterterrorism counsel at Human Rights Watch, said, “It would be contrary to the principles of the criminal justice system for the attorney general to say he believes a very serious crime has been committed and then to do nothing about it.”

Charles D. Stimson, who served as the Defense Department’s top official on detainee affairs from 2004 to 2007 and is now a senior legal fellow at the conservative Heritage Foundation, said the statements “certainly will increase the pressure on Holder to mount some kind of investigation.”

In addition to domestic political pressures, the United States appears to have a legal obligation as a party to the international Convention against Torture to follow up on the torture statements. That treaty requires signatory states to conduct a “prompt and impartial investigation, wherever there is reasonable ground to believe that an act of torture has been committed in any territory under its jurisdiction.”

The Bush administration placed its interrogation operations offshore, at the American base in Cuba and at secret C.I.A. sites, and officials have sometimes argued that they were not on territory under American jurisdiction. But that assertion has been eroded by court decisions concerning the Guantánamo detention center, and it is unlikely that the Obama administration would use such a loophole to avoid the torture convention’s effect.

“There’s a moral, legal and practical obligation of the United States to follow this allegation in good faith wherever it leads,” said Juan E. Méndez, a veteran human rights lawyer who is president of the International Center for Transitional Justice in New York.

Where such an inquiry might lead is an unsettling question for departing Bush administration officials, who have long worried that aggressive policies could make them vulnerable to civil or criminal liability.

If rank-and-file interrogators are protected by the Justice Department’s assurance that their actions were legal, what about the lawyers who gave the assurances? What about the senior officials, including President Bush, who approved the use of waterboarding and other such tactics?