The report said that several senior Justice Department Criminal Division officials raised concerns with the National Security Council in 2003 about the military’s treatment of detainees, but saw no changes as a result of their complaints.

John Ashcroft, the former attorney general, declined to be interviewed by the inspector general’s office of the department he had headed, an unusual refusal and one that hampered investigators’ attempts to learn of discussions inside the council, the report said.

A Pentagon spokesman had no immediate comment on the report.

The inspector general’s office started its investigation in late 2004, following widespread public attention to the question of detainee treatment spurred by graphic photographs of prisoners at the Abu Ghraib prison in Iraq.

The American Civil Liberties Union, through a lawsuit, also unearthed numerous internal e-mail messages from the bureau about agents’ complaints of rough interrogation tactics at Guantánamo Bay, which proved central in the Justice Department’s review.

The investigation examined about a half-million documents and included surveys of 1,000 F.B.I. agents regarding their experiences with interrogation tactics by military and C.I.A. interrogators, as well as interviews with hundreds of other bureau personnel, officials said. The investigation centered on the accounts of what the agents witnessed in the treatment of prisoners at Guantánamo Bay, in Iraq and in Afghanistan, and how those complaints were handled. The Justice Department’s inspector general does not have jurisdiction over the Pentagon.

The bulk of the report was completed last year, but its public release by the inspector general was bottled up for months because of concerns from the Defense Department about the disclosure of sensitive information centering on interrogation tactics. The final report from the inspector general, unlike some earlier terrorism investigations, was released with relatively few blacked-out sections.

The bureau stationed agents at Guantánamo Bay and other military detention sites to assist in the questioning of detainees taken into custody after the terror attacks of Sept. 11, 2001, but the rough tactics by military interrogators soon became a major source of friction between the bureau and sister agencies. Agents complained to superiors beginning in 2002 that the tactics they had seen in use yielded little actual intelligence, prevented them from establishing a rapport with detainees through more traditional means of questioning, and might violate bureau policy or American law.