Another memo dated January 21 2004 which discusses a practice by some interrogators of impersonating FBI agents, mentions the deputy defence secretary, Paul Wolfowitz, suggesting that the policy was approved high in the Pentagon.

"This technique, and all of those used in these scenarios, was approved by the Dep Sec Def," reads the memo.

The FBI agents believed that by impersonating agents, military interrogators were trying to exploit the rapport the agency had established with some inmates.

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In a document dated last July, an FBI agent at Guantánamo tells his supervisors he was upset by the interrogation methods used by military interrogators and government contractors. "I did observe treatment that was not only aggressive, but personally very upsetting," the memo said.

The treatment included chaining detainees to the floor in the foetal position, or subjecting them to extreme heat or cold. "Most times they had urinated or defecated on themselves and had been left there for 24 hours or more," one memo said.