Major General Antonio Taguba also left open the possibility that members of the CIA as well as armed forces personnel and civilian contractors were culpable in the abusive treatment at the Abu Ghraib prison.
"A few soldiers and civilians conspired to abuse and conduct egregious acts of violence against detainees and other civilians outside the bounds of international laws and the Geneva Convention," he told the Senate Armed Services Committee in Washington.
Without mentioning names, Maj Gen Taguba pointed to Brigadier General Janis Karpinski of the 800th Military Police Brigade for failed command leadership. Brig Gen Karpinski, who had command of military prisons in Iraq, has been suspended in connection with the abuse. She has not been charged.
"These acts of abuse were not the spontaneous actions of lower enlisted personnel they were clearly planned and suggested by others," said Senator Carl Levin, the panel's senior Democrat.
Questions about ultimate responsibility for control of the Abu Ghraib prison produced a disagreement between Stephen Cambone, the undersecretary of defence for intelligence, and Maj Gen Taguba, who said it had been turned over to military intelligence.
Mr Cambone said that was incorrect, and it resided with the military police.
In a further disagreement, Maj Gen Taguba said it was against Army rules for intelligence troops to involve MPs in setting conditions for interrogations. Mr Cambone said he believed it was appropriate for the two groups to collaborate.
Maj Gen Taguba also told the committee his investigation had not found "any order whatsoever, written or otherwise," that directed the military police to co-operate with intelligence forces at the prison.
Regardless of any disagreements, Mr Cambone and others told the panel that troops in Iraq were under orders to abide by the Geneva Conventions, which dictate terms for humane treatment of prisoners. "An order to soften up a detainee would not be a legal order, would it?" asked Senator Pat Roberts.
"No sir," replied Lieutenant General Lance F Smith, deputy director of the US Central Command.
Maj Gen Taguba said his investigators had been told about participation by "other government agencies [a euphemism for the CIA] or contractors" in the abuse. Mr Cambone, too, was asked whether he had any knowledge of CIA involvement: "There were people brought by agency personnel to that place. ... There may have been interrogations conducted by the agency personnel while they were there," he said.