The US army has thrown out the conviction of the only officer court martialled in the Abu Ghraib scandal, ending the four-year investigation and drawing complaints from human rights activists of a Pentagon whitewash.

Barring any new information, the decision means no officers or civilian leaders will be held criminally responsible for the prisoner abuse - which included the photographing of Iraqi prisoners in painful and sexually humiliating positions - which embarrassed the military and inflamed the Muslim world.

Lieutenant Colonel Steven Jordan was cleared of any criminal wrongdoing by Major General Richard Rowe, the Washington military district chief. He was instead given an administrative reprimand. Jordan, 51, was acquitted at his court martial in August of charges that he failed to supervise the 11 lower-ranking soldiers convicted for their roles in the abuse.

But he was found guilty of disobeying an order not to talk about the investigation, and the jury recommended a criminal reprimand, the lightest possible punishment. Jordan told the Associated Press yesterday that he felt victimised by press coverage that seemed eager for an officer to be blamed for the abuse. He also said he agreed that there were both enlisted soldiers and officers responsible for the abuse who escaped prosecution.

"Everybody that's seen all the evidence and looked at it, or the lack of it, realises that Steve Jordan had nothing to do with abusing detainees at Abu Ghraib," Lt Col Jordan said. Major Kris Poppe, the defence lawyer, argued Lt Col Jordan "faced these very serious charges for a long period of time, that he had been found not guilty of any offence related to the abuse of detainees, and that he had a stellar record."

Rowe agreed. "In light of the nature of the offence that Jordan had been found guilty of committing and the substantial evidence in mitigation at trial and in post-trial matters submitted by defence counsel, Rowe determined that an administrative reprimand was a fair and appropriate disposition of the matter," Joanna Hawkins, a military spokeswoman, said in a statement.

But human rights advocates said the case did not go higher up the chain of command and said the decision sent a troubling message.

"It could not be more clear that prisoner abuse in Iraq and Afghanistan resulted from policies and practices authorized by high-level officials, including military and civilian leaders," said Hira Shamsi, of the National Security Project of the American Civil Liberties Union.

"Although the abuse was systemic and widespread, the accountability for it has been anything but."

Mila Rosenthal, a director for research and policy for Amnesty International USA, said: "I think we're emboldening dictators and despots around the world. We're saying that it's OK to allow these kinds of abuses to flourish."