A US Senate committee has accused the former defence secretary, Donald Rumsfeld, of being directly responsible for the abusive interrogations of detainees at Abu Ghraib in Iraq, Afghanistan and Guantánamo Bay.

After an 18-month investigation, the Senate's armed services committee concluded that Rumsfeld's approval of aggressive interrogation methods in December 2002 was a direct cause of abuses that began in Guantánamo and spread to Afghanistan and Iraq. They culminated in the Abu Ghraib scandal in 2003, where Iraqi detainees were found to have been forced into naked pyramids, sexually humiliated and threatened by dogs.

The Bush administration insisted the abuses had been the result of a few "bad apples" and that those responsible would be held accountable. The committee found neither those statements to be true.

"The abuses at Abu Ghraib, Gitmo [Guantánamo] and elsewhere cannot be chalked up to the actions of a few bad apples," said the Democratic chair of the committee, Carl Levin. "Attempts by senior officials to portray that to be the case while shrugging off any responsibility are both unconscionable and false."

No other congressional report has pointed the finger of blame so squarely at Bush and his senior advisers.

In hearings in June and September, the committee heard testimony that allowed it to piece together the chronology of events leading up to the Abu Ghraib abuses. It focused its attentions on Sere, a training system used to prepare US soldiers for aggressive interrogations so that they might endure if captured overseas.

The techniques were never intended to be used by US interrogators against their detainees. But in February 2002, in the wake of the 9/11 attacks, Bush determined that the Geneva Conventions should not apply to terror suspects.

Following that ruling, techniques used in Sere training were applied against US detainees, and Rumsfeld gave his approval that December.