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A military review concluding that blame for wartime prisoner abuse lay mostly with low- and midlevel soldiers has failed to quiet critics who say Defense Secretary Donald H. Rumsfeld (search) and other senior leaders should be held to account.

There's a big problem, one senator said Thursday, when investigators are "in the chain of command of the officials whose policies and actions they are investigating."

In presenting the results of his investigation to the Senate Armed Services Committee, Vice Adm. Albert T. Church (search) said Thursday there was no single explanation for the mistreatment of Iraqi, Afghan and other prisoners under the control of U.S. military personnel. And he said there was no evidence to indicate Rumsfeld or any other senior civilian or military authority directed, approved or encouraged a policy of prisoner abuse.

Church said he did not focus on senior official accountability because that had been assessed in earlier investigations. But that did not satisfy some senators who believe Rumsfeld should take blame for creating an environment in which mistreatment of prisoners might appear to be tolerated.

"So there's been no assessment of accountability of any senior officials, either within or outside of the Department of Defense, for policies that may have contributed to abuses of prisoners," said Sen. Carl Levin of Michigan, the committee's top Democrat.

"I can only conclude that the Defense Department is not able to assess accountability at senior levels, particularly when investigators are in the chain of command of the officials whose policies and actions they are investigating," he added.

Church, a former Navy inspector general, is now director of the Navy staff at the Pentagon. A 21-page summary of his findings was made public, but the Pentagon said it did not intend to release the full report. Church said many of the details underlying his conclusions are classified.

Asked later at a Pentagon news conference whether he saw any reason not to release his full report publicly, Church said that as long as classified information was removed, "that would be fine."

The human rights group Amnesty International USA (search) criticized Church's report as being too easy on top officials.

"The Church report should be published in full, and the record of senior officials thoroughly examined by an independent commission of inquiry," said Alexandra Arriage, Amnesty International's director of government relations.

Church's report, which is at least the sixth senior-level assessment of issues related to the prisoner abuse at Abu Ghraib (search) and elsewhere, also failed to put to rest a political debate over the White House's decision not to afford Geneva Convention (search) protections to some enemy fighters, including the Taliban army that fought in Afghanistan.

Sen. John McCain, R-Ariz., who was a prisoner of war in Vietnam, questioned the wisdom of deciding that certain categories of enemy fighters should be ineligible for Geneva Convention protections.

"I worry, admiral, very much that if we decide that a certain country's military personnel are not eligible for treatment under a convention that we signed," McCain said, "then wouldn't it be logical to expect then they would declare, as the North Vietnamese did, that American prisoners are not eligible for protection under the Geneva Conventions?"

Church said that he believes President Bush made the right call on that issue.

Church's investigation focused on the Pentagon's development of interrogation policies and techniques and the extent to which that process could be linked to the sexual humiliation of Iraqi prisoners at Abu Ghraib and other acts of mistreatment documented in Iraq and Afghanistan.

"Even in the absence of a precise definition of 'humane' treatment, it is clear that none of the pictured abuses at Abu Ghraib bear any resemblance to approved policies at any level," Church concluded.

Earlier military investigations of prison abuse also refrained from harsh criticism of senior leaders, in line with the Pentagon's explanation that the dozens of incidents of confirmed prisoner abuse were the work of low-level soldiers and a few inattentive midlevel officers.

Some Republicans chafed at the criticism of the U.S. military.

"If our guys want to poke somebody in the chest to get the name of a bomb maker so they can save the lives of Americans, I'm for it," said Sen. Jim Talent, R-Mo. "I don't need an investigation to tell me that there was no comprehensive or systematic use of inhumane tactics by the American military, because those guys and gals just wouldn't do it."

Church acknowledged that he could not point to a clear explanation of why prisoners were abused.

"If approved interrogation policy did not cause detainee abuse, the question remains: What did?" he wrote in the summary. He offered three contributing factors:

--Most of the documented cases of abuse happened not in prisons but on the battlefield at what the military calls the "point of capture," where Church said "passions often run high" as soldiers find themselves face-to-face with captured fighters. "Discipline was lacking in some instances," he wrote.

--Lower- and midlevel commanders failed to react to early warning signs of abuse. Church said he could not provide details because they are classified secret, but he said such warning signs were present, particularly at Abu Ghraib, and should have prompted action to prevent further abuse.

--There was a breakdown of good order and discipline in some field units. "This breakdown implies a failure of unit-level leadership to recognize the inherent potential for abuse," to recognize and alleviate stress on troops handling prisoners, and to provide oversight.