A US general who investigated abuses by US soldiers against Iraqis held in jail has blamed failures of leadership for allowing the situation to develop.
Taguba: Catapulted from relative obscurity since his report leaked
Maj Gen Antonio Taguba told a Senate committee there had been no training or supervision, and a lack of discipline.
A few soldiers and civilians conspired to commit "incomprehensible" acts at Abu Ghraib prison near Baghdad and capture them on film, he said.
But he found no evidence of a policy or specific order directing the abuse.
Gen Taguba, who looked at a number of jails in Iraq, found "numerous incidents of sadistic, blatant, and wanton criminal abuses" by prison guards at Abu Ghraib in his report.
The Senate hearing comes as the authorities consider whether to make public further images of alleged abuse - including shots of what a defence department spokesman termed "inappropriate behaviour of a sexual nature".
Photographs showing naked Iraqi detainees being humiliated and maltreated first started to surface just under two weeks ago, sparking shock and anger - especially in the Arab world.
They have fuelled further criticism of the US-led occupation of Iraq just weeks before authority is to be handed over to an interim Iraqi administration.
Correspondents say the scandal has grown ever more embarrassing for George W Bush ahead of this year's US presidential election, and appears to be having a severe effect on the American public's view of the entire Iraq war.
Opening the latest in a series of hearings, Senate Armed Forces Committee chairman John Warner said the abuse of detainees had damaged America's image abroad and there must now be a full accounting.
Gen Taguba told the committee: "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."
Detainees at Abu Ghraib prison were forced to commit sexual acts, were threatened with torture, rape or attack by dogs, and were hidden from Red Cross visits, "in violation of international law", according to his 53-page report which focused on blocks where interrogation took place.
Asked directly what had led to the abuses, Gen Taguba told the committee: "Lack of discipline, no training whatsoever, and no supervision. Supervisory omission was rampant."
His testimony comes as the latest nationwide opinion poll suggests, for the first time, that a majority of Americans now believe the Iraq war was not worth fighting.
The report was originally kept secret but later leaked to the media.
The general was ordered on 24 January to conduct an investigation into mistreatment of Iraqi prisoners. It was completed on 3 March, the Pentagon said, but by 4 May the defence secretary had still not read it fully.
Gen Taguba agreed with one senator that the abuse appeared to be part of a systematic effort to soften prisoners up before their interrogation, but said:
"I did not find any 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. I
believe that they collaborated with several MI [military
intelligence] interrogators at the lower level, based on the
conveyance of that information through interviews and written
The findings of the report have led to seven US army reservists facing charges over their alleged treatment of detainees.
After the allegations emerged, Brig Gen Janis Karpinski - who was in charge of the military police unit that ran Abu Ghraib and other jails when the abuses were allegedly committed - was suspended. She is not among those charged.
Gen Karpinski, from the army reserve, has said she was "sickened" by the images from the Baghdad jail.
The under-secretary of defence for intelligence, Stephen Cambone, told the Senate committee there had been a clear breakdown of observance of the Geneva Conventions on the treatment of prisoners in Iraq.
However he said it was hard to explain how the abuse had happened.