This prisoner and other "ghost detainees" were hidden largely to prevent the International Committee of the Red Cross from monitoring their treatment, and to avoid disclosing their location to an enemy, officials said.

Maj. Gen. Antonio M. Taguba, the Army officer who in February investigated abuses at the Abu Ghraib prison, criticized the practice of allowing ghost detainees there and at other detention centers as "deceptive, contrary to Army doctrine, and in violation of international law."

This prisoner, who has not been named, is believed to be the first to have been kept off the books at the orders of Mr. Rumsfeld and Mr. Tenet. He was not held at Abu Ghraib, but at another prison, Camp Cropper, on the outskirts of Baghdad International Airport, officials said.

Pentagon and intelligence officials said the decision to hold the detainee without registering him - at least initially - was in keeping with the administration's legal opinion about the status of those viewed as an active threat in wartime.

Seven months later, however, the detainee - a reputed senior officer of Ansar al-Islam, a group the United States has linked to Al Qaeda and blames for some attacks in Iraq - is still languishing at the prison but has only been questioned once while in detention, in what government officials acknowledged was an extraordinary lapse.

"Once he was placed in military custody, people lost track of him," a senior intelligence official conceded Wednesday night. "The normal review processes that would keep track of him didn't."

The detainee was described by the official as someone "who was actively planning operations specifically targeting U.S. forces and interests both inside and outside of Iraq."

But once he was placed into custody at Camp Cropper, where about 100 detainees deemed to have the highest intelligence value are held, he received only one cursory arrival interrogation from military officers and was never again questioned by any other military or intelligence officers, according to Pentagon and intelligence officials.

The Pentagon's chief spokesman, Lawrence Di Rita, said Wednesday that officials at Camp Cropper questioned their superiors several times in recent months about what to do with the suspect.

But only in the last two weeks has Mr. Rumsfeld's top aide for intelligence policy, Stephen A. Cambone, called C.I.A. senior officials to request that the agency deal with the suspect or else have him go into the prison's regular reporting system.

Mr. Di Rita referred questions about the prisoner's fate to the C.I.A.

A senior intelligence official said late Wednesday that "the matter is currently under discussion."

In July 2003, the man suspected of being an Ansar al-Islam official was captured in Iraq and turned over to C.I.A. officials, who took him to an undisclosed location outside of Iraq for interrogation. By that fall, however, a C.I.A. legal analysis determined that because the detainee was deemed to be an Iraqi unlawful combatant - outside the protections of the Geneva Conventions - he should be transferred back to Iraq.

Mr. Tenet made his request to Mr. Rumsfeld - that the suspect be held but not listed - in October. The request was passed down the chain of command: to Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, then to Gen. John P. Abizaid, the commander of American forces in the Middle East, and finally to Lt. Gen. Ricardo S. Sanchez, the ground commander in Iraq. At each stage, lawyers reviewed the request and their bosses approved it.

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