"The whole thing is disgusting and it's hard to believe that this actually is taking place in a military facility," said Sen. Dianne Feinstein.
"I expected that these pictures would be very hard on the stomach lining and it was significantly worse than anything that I had anticipated," said Sen. Ron Wyden. "Take the worse case and multiply it several times over."
Several senators, speaking on condition of anonymity, said photos of sexual intercourse were among the images that Defense Department officials screened for lawmakers in a top-secret room in the Capitol. At least some of them appeared to depict consensual sex involving U.S. military personnel, they added.
Others showed military dogs snarling at cowering prisoners, as well as shots of Iraqi women commanded to expose their breasts, these senators said.
Some key senators thought the pictures should remain secret -- and not just because they are evidence in a criminal investigation, reports CBS News Correspondent David Martin.
"It diminishes these photos from inspiring the enemy from inflicting further damage, harm, otherwise on the members of the armed forces of the United States,'' said Sen. John Warner, R-Va.
The private screening marked the latest turn in a scandal that has rocked the Bush administration and apparently led to thewho said they were avenging the abuse at the Abu Ghraib prison.
"I don't know how the hell these people got into our army," said Ben Nighthorse Campbell after viewing the images.
"There were several pictures of Iraqi women who were disrobed or putting their shirts up," he said. "They were not smiling in the pictures, that's for sure. But it didn't look like they had been beaten or hurt."
He also said there were several pictures with dogs. "Iraqis were against the wall and you could see that the dogs were pretty much terrorizing them because the dogs were snarling and crouching like they were about to attack," he added.
The Pentagon and Congress have already begun investigating the abuse, and one of the early questions has revolved around who bears responsibility: the relatively small number of lower-ranking military personnel seen in some of the photos or officers higher up the chain of command.
"In one particular still photo among troops that are in a hallway, where you've seen the clump of people tied together on the floor, we counted seven or eight troops," said Sen. Bill Nelson, D-Fla. "Now you can't tell me all of this is going with seven or eight Army privates. ... Where did that failure of the command and control occur?"
Officials said Pentagon officials carried three discs with them to the Capitol, containing about 1,800 still images as well as an undisclosed number of videos. Maj. Gen. Antonio Taguba, who conducted the Army's first investigation into the abuse, told Congress on Tuesday that he believed the pictures were taken by military personnel using their personal digital cameras.
Taguba also testified that the mistreatment resulted from faulty leadership, a "lack of discipline, no training whatsoever and no supervision" of the troops, members of a U.S. Army Reserves unit called up for duty in postwar Iraq. He said he found no evidence of culpability further up the chain of command, or of systematic abuse in other prisons.
At the same time, Taguba said tactical control of Abu Ghraib had been turned over to a military intelligence unit. That raised questions about additional possible culpability, and the Army is in the midst of another investigation to determine the role played by intelligence officers or enlisted personnel who were present at the prison.
President George W. Bush has apologized for the abuse, and Defense Secretary Donald H. Rumsfeld told Congress last week he accepted responsibility.
At the same time, the vivid evidence of abuse has prompted several Democratic lawmakers to raise fresh questions about administration policy in Iraq.
At a hearing a few hours before the pictures were screened, Rumsfeld defended military interrogation techniques, rejecting complaints that they violate international rules and may endanger Americans taken prisoner.
Rumsfeld told a Senate committee that Pentagon lawyers had approved methods such as sleep deprivation and dietary changes as well as rules permitting guards to make prisoners assume stressful positions.
Gen. Richard Myers, chairman of the Joint Chiefs of Staff, noted that the rules require prisoners to be treated humanely at all times.
But Sen. Richard Durbin said some of the approved techniques "go far beyond the Geneva Convention," a reference to international rules governing the treatment of prisoners of war.
Durbin noted that one American GI was missing in Iraq, his whereabouts unknown. Given the circumstances, he asked Rumsfeld, "wouldn't it help if there was clarity from you and from this administration that we would abide by the Geneva Convention when it comes to civilian and military detainees unequivocally?"
Rumsfeld replied that the Geneva Convention applies to all prisoners held in Iraq, but not to those held in Guantanamo Bay, where detainees captured in the global war on terror are held.
In Baghdad on Wednesday, Brig. Gen. Mark Kimmitt announced that two more American soldiers have been ordered to stand trial in the prisoner abuse scandal, although no date for the courts martial have been set.
Sgt. Javal Davis, 26, of Maryland and Staff Sgt. Ivan L. "Chip" Frederick II of Buckingham, Virginia were ordered to undergo a general court martial, Kimmitt said.
Both men were charged with conspiracy to maltreat detainees, dereliction of duty for failing to protect detainees from abuse, maltreatment of detainees, and other counts.
In writings, Frederick has suggested that intelligence officers urged the abuse. In an interview with reporter Brian Maass of Denver CBS station KCNC, Army Pvt. Lynndie England, a 21-year-old who features in some of the abuse photographs, also.
"I was instructed by persons in higher rank to stand there and hold this leash and look at the camera," she said.