The US military claimed in the note that Dr Izmerly, a distinguished chemistry professor arrested after US tanks encircled his villa, had died of “brainstem compression”.

Dr Izmerly’s sudden death after 10 months in American custody left his family stunned, not least because three weeks earlier they had visited him in the US prison at Baghdad airport. His 23-year-old daughter, Rana, recalled that he had seemed in “good health”.

The family commissioned an independent Iraqi autopsy. Its conclusion was unambiguous: Dr Izmerly had died because of a “sudden hit to the back of his head”, Faik Amin Baker, the director of Baghdad hospital’s forensic department, certified. . . .

The apparent murder of a “high-value” detainee, held as part of the search for weapons of mass destruction, is another blow for the Bush administration, still reeling from the Abu Ghraib jail abuse scandal.

Dr Izmerly was on the coalition’s original “200 list” of suspects from Saddam Hussein’s regime, and his death happened just two weeks after the US military began its own secret inquiry into the prison west of Baghdad. Last Friday the Pentagon admitted it was now investigating eight more suspected murders.

Several prisoners have been found to have died before or during interrogation. They include Major General Abed Hamed Mowhoush, a former commander of Iraq’s air defences, who died last November during interrogation at Qaim.

The original US autopsy said he had died of a heart attack. It now appears he was suffocated during interrogation when a CIA officer put him in a sleeping bag and sat on him.

Last night the family of Dr Izmerly were in little doubt he had been murdered in US custody. The reasons for his death were covered up, they believe.

“This was not natural,” Rana told the Guardian yesterday, in the first interview given by the family since his death. “The evidence is clear. It suggests the Americans killed him and then tried to hide what they had done. I will hate Americans and British people for the rest of my life. You are democrats. You said you were coming to bring democracy, and yet you kill my father. By accepting your governments, you accept what they do here in Iraq.