Major General Antonio Taguba told a Senate committee on Tuesday that responsibility for the crimes should be accepted by "the brigade commander on down".
Asked how the abuse happened, he said: "Lack of discipline, no training whatsoever, and no supervision. Supervisory omission was rampant."
But he said he found no "evidence of a policy or a direct order given to these soldiers to conduct what they did".
"I believe that they did it on their own volition and I believe that they collaborated with several (military intelligence) interrogators at the lower level."
At the Pentagon's insistence, Under Secretary of Defence Stephen Cambone, who is in charge of intelligence, and other Pentagon officials also appeared with Taguba.
Democrats on the committee were irked that the Pentagon balked at plans for the general to testify by himself, calling it an "attempt to dilute Taguba's testimony".
The scandal, which US officials acknowledge has threatened to undermine their policy goals in Iraq and the broader Middle east, broke as public support for the Iraq war was already declining.
A Today/Gallup Poll released on Monday found only 44% believed the war was worthwhile.
In a poll taken a month ago, 50% said it was worth going to war in Iraq. A year ago, 73% thought the war was worthwhile.