Senior leaders did not establish clear guidelines on permissible techniques for interrogating various categories of prisoners held at Abu Ghraib and elsewhere in Iraq, the report said.
High-level commanders failed to shift resources to an understaffed and ill-trained prison detention unit once it became apparent that the system was out of control, the report said.
The findings were presented at a Pentagon news conference by James Schlesinger, the former secretary of defense who headed a four-person commission created last May by Defense Secretary Donald H. Rumsfeld.
"There was chaos at Abu Ghraib," Schlesinger said, and it was due in part to the fact that the prison was a regular target of shelling by an Iraqi insurgency not adequately anticipated by U.S. leaders.
The report said the direct responsibility lay with soldiers and commanders in the field rather than in Washington.
"There was direct responsibility for those activities on the part of the commanders on the scene up to the brigade level, because they did not adequately supervise what was going on at Abu Ghraib," Schlesinger said. "There was indirect responsibility at higher levels, in that the weaknesses at Abu Ghraib were well-known and that corrective action could have been taken and should have been taken."
He said Rumsfeld's office could be faulted for inadequate supervision, but he strongly objected to the suggestion that Rumsfeld should step down from his post.
"His resignation would be a boon to all of America's enemies," Schlesinger said.
Asked later about the culpability of senior military commanders, Schlesinger said "they were not focused on the detention operations," but even so they should not be forced to resign or be punished. He referred specifically to Gen. Richard Myers, chairman of the Joint Chiefs of Staff, and Lt. Gen. Ricardo Sanchez, who was the top U.S. commander in Iraq during the period in question.
The mistreatment of prisoners, described by the commission as "acts of brutality and purposeless sadism," would have been avoided with proper training, leadership and oversight, the report said.
In most cases, the abuse was not carried out with the purpose of achieving intelligence from prisoners, Schlesinger said.
"There were freelance activities on the part of the nightshift at Abu Ghraib," he said. "It was a kind of 'Animal House' on the night shift."
The report did not suggest that Rumsfeld ordered any of the abuses or did anything to encourage them. But it indicated that his policies created some confusion at lower levels of the military.
"The abuses were not just the failure of some individuals to follow known standards, and they are more than the failure of a few leaders to enforce proper discipline," the report said. "There is both institutional and personal responsibility at higher levels."
The commission was particularly critical of Sanchez and other commanders.
"We believe Lt. Gen. Sanchez should have taken stronger action in November when he realized the extent of the leadership problems at Abu Ghraib," the report said. It concluded that he "failed to ensure proper staff oversight" of detention and interrogation operations.
Sanchez also takes a portion of the blame in a separate Army investigation which looked specifically at the role of military intelligence soldiers.
That probe has been completed and is expected to be publicly released as early as Wednesday. It is expected to say that at least two dozen lower-ranking military intelligence soldiers, as well as civilian contractors, were responsible for the abuses, which were depicted in photographs and videos taken by U.S. soldiers.
The Schlesinger commission made no recommendations about disciplinary action against any civilian or military officials.
The question of how high responsibility for the abuse goes continues to be one of the central unanswered questions in the scandal ? and it is key to the ongoing criminal cases against several low-ranking military police soldiers charged with mistreatment of detainees at Abu Ghraib.
Six military police accused of abusing prisoners at the prison near Baghdad insist they were following orders from military intelligence officers and civilian contractors. A seventh soldier pleaded guilty May 19 to taking pictures of naked prisoners and was sentenced to a year in prison.
None of the investigations has found that Rumsfeld or Myers ordered or encouraged any mistreatment of prisoners. In May, Rumsfeld told the House and Senate that as secretary of defense "I am accountable" for the events at Abu Ghraib and he issued "my deepest apologies" to the Iraqis who were abused.
The Schlesinger commission interviewed Rumsfeld twice during its investigation, which began in May. The three other commission members are former defense secretary Harold Brown, former Republican Rep. Tillie Fowler of Florida, and retired Air Force Gen. Charles Horner.
When he chartered the commission, Rumsfeld told its members that he wanted independent advice on a wide range of issues related to the abuse allegations. "I am especially interested in your views on the cause of the problems and what should be done to fix them," he wrote at the time.