The question of whether those methods amounted to torture is a historically and legally momentous issue that has been debated for more than a decade inside and outside the government. The Justice Department’s Office of Legal Counsel wrote a series of legal opinions from 2002 to 2005 concluding that the methods were not torture if used under strict rules; all the memos were later withdrawn. News organizations have wrestled with whether to label the brutal methods unequivocally as torture in the face of some government officials’ claims that they were not.

In addition, the United States is a signatory to the international Convention Against Torture, which requires the prompt investigation of allegations of torture and the compensation of its victims.

Like the still-secret Senate interrogation report, the Constitution Project study was initiated after President Obama decided in 2009 not to support a national commission to investigate the post-9/11 counterterrorism programs, as proposed by Senator Patrick J. Leahy, Democrat of Vermont, and others. Mr. Obama said then that he wanted to “look forward, not backward.” Aides have said he feared that his own policy agenda might get sidetracked in a battle over his predecessor’s programs.

The panel studied the treatment of prisoners at Guantánamo Bay, in Afghanistan and Iraq, and at the C.I.A’s secret prisons. Staff members, including the executive director, Neil A. Lewis, a former reporter for The New York Times, traveled to multiple detention sites and interviewed dozens of former American and foreign officials, as well as former detainees.

Mr. Hutchinson, who served in the Bush administration as chief of the Drug Enforcement Administration and under secretary of the Department of Homeland Security, said he “took convincing” on the torture issue. But after the panel’s nearly two years of research, he said he had no doubts about what the United States did.

“This has not been an easy inquiry for me, because I know many of the players,” Mr. Hutchinson said in an interview. He said he thought everyone involved in decisions, from Mr. Bush down, had acted in good faith, in a desperate effort to try to prevent more attacks.

“But I just think we learn from history,” Mr. Hutchinson said. “It’s incredibly important to have an accurate account not just of what happened but of how decisions were made.”