Gen. Kern testified that there were dozens possibly up to one hundred detainees that the CIA avoided registering. Gen. Fay estimated the number of ghost detainees at two dozen. Documentation on these detainees is virtually non-existent. The officers testifying at the hearing were unable to get the documents they needed from the CIA to fully investigate this issue. In its defense, the CIA says it's conducting it's own investigation into the issue.
It's important to note that hiding detainees is a violation of the Geneva Conventions, which applied to Iraqi detainees. A delay in registration is legal if militarily necessary, but the Generals testifying agreed that these cases went far beyond the delay allowed by law.
The Washington Post is reporting that one intelligence official says that the CIA only approved the lack of registration for a few detainees and the rest were the independent actions of operatives in the field any may have been a willful delay of registration and not an intent to avoid registering the detainees altogether. Another source claims that avoiding registration was intended to keep other detainees from finding out that a detainee had been captured so that they couldn't coordinate their stories.
Apparently, the CIA had permission of Brig. Gen. Fast, the senior intelligence officer in Iraq, to bring detainees to Abu Ghraib but there was no written agreement between the CIA and the military on the handling of the detainees. The military says it expected the CIA to follow military procedures which would include registering detainees. However, while it may have been the CIA that decided not to register the detainees, it was the military personnel that had to carry out - or not carry out - the registration procedure. Apparently, Col. Pappas, the intelligence officer overseeing interrogations at Abu Ghraib, questioned the practice of hiding detainees and was told by Col Boltz, a senior intelligence officer to cooperate. Nevertheless, Col. Pappas was criticized by the generals testifying for not challenging the practice.
Note: The CIA response that most "ghost detainees" were made so by the independent action of field operatives sounds an awful lot like the claim that the abuse at Abu Ghraib was the action of a handful of rogue soldiers. The explanation that they hid detainees to keep them from coordinating their stories also seems quite weak to me. It implies that the military had no way of preventing communication between detainees and that seems unlikely to me. Additionally, while the Pentagon is pointing the finger at the CIA, it would be wise to remember that Def. Sec. Rumsfeld has already admitted that he approved not registering one detainee, though Rumsfeld claimed that for some reason this didn't violate the Geneva Conventions, a position later reversed by the administration.
Culpability of Senior Officers
Neither the Fay or Schlesinger reports find any officers above the rank of brigade commander culpable for the abuse of detainees. However, both investigations laid some responsibility on senior officers and civilians. For example, the Fay Report found "that 'higher headquarters,' including the U.S. Central Command, the Pentagon and the Defense Intelligence Agency, applied pressure for 'timelier, actionable intelligence," affecting decision-making at the interrogation facility at Abu Ghraib'." (LA Times)
During the hearing, Sen. Reed's (D-RI) question was on point: ""Why would all these people not follow Army regulations, not report violations to the Geneva Conventions, wait months to inform commanders of vital information? I don't think you've reached that, to me, basic question of what went on out there." Senators from both sides of the aisle pressed this issue sufficiently enough that Gen. Kern agreed to revisit their findings to determine whether or not Lt. Gen. Sanchez, who was the top commander in Iraq, or his senior staff should face prosecution. One officer was singled out as an example - Col. Marc Warren, a lawyer under Gen. Sanchez' command. Apparently, Col. Warren knew about abuse seen and reported by Red Cross inspectors but didn't report the abuse for over a month, saying he didn't believe the reports. Other senior officers identified by Sen. Kennedy as worthy of attention include U.S. Central Command chief Army Gen. John P. Abizaid; Air Force Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff; Deputy Defense Secretary Paul D. Wolfowitz; and Defense Secretary Donald H. Rumsfeld.
During the House hearing, former defense secretaries, James R. Schlesinger and Harold Brown both said that failures on the part of two undersecretaries of defense, Douglas Feith and David Chu, as well as the Pentagon's general counsel William Haynes, contributed to the abuses at Abu Ghraib. Despite the failure of Rumsfeld's top staff, both former secretaries praised Rumsfeld and asserted that he should not resign in response to the abuse or failures of his staff.
However, former Secretary Brown did blame the Bush administration for "failing to plan for what actually happened after the overthrow of Saddam Hussein." (NYT). Brown stated that it was up to the voters to hold the administration responsible.