Muhammad Munim al-Izmerly, 65, who allegedly experimented with poison on prisoners during Saddam Hussein's rule, is among at least 96 detainees who have died in US custody in Iraq.
Questions have surrounded his death ever since his body was dropped off at a Baghdad hospital in February 2004, two weeks after he died.
When it first came to light in press reports last May, the US military, newly under fire for prisoner abuse in Iraq, refused to answer queries about the Egyptian-born chemist's death.
Now, months later, the army says an investigation has begun.
"The case was initially closed, but after further investigative review a determination was made to reopen the investigation," Army spokesman Christopher Grey said.
"The case was initially closed, but after further investigative review a determination was made to reopen the investigation"
The Pentagon would say nothing about the timetable or thrust of the inquiry. But Rod Barton, an Australian member of the CIA-led teams that questioned al-Izmerly and other weapons scientists, says such prisoners may have been beaten during the futile US hunt for banned arms in Iraq.
When al-Izmerly's body was delivered to Al-Kharkh Hospital, the Americans enclosed a death certificate saying he died of "brain-stem compression", without saying what caused it, Britain's Guardian newspaper reported after viewing the document last year.
A subsequent Iraqi autopsy determined he was killed by a blunt trauma injury, a blow to the head, Iraqi doctors told Baghdad reporters.
The scientist had been in US detention since April 2003.
Al-Izmerly's family was allowed to visit him in January 2004 at the Baghdad airport, where he was believed held at Camp Cropper, a US military detention centre for high-value detainees.
The US military has been under
fire for prisoner abuse in Iraq
A month later the family was notified by the Red Cross he was dead. His son, Ashraf, 22, told reporters that when he went to the hospital morgue to claim the remains, zipped up in a US body bag, he saw an injury to the head. The dated death certificate indicated the Americans had held the body for 17 days.
The family commissioned an autopsy, which found the cause of death to be a blow to the head, the reports from Baghdad said.
"It was definitely a blunt trauma injury," the Los Angeles Times was told by Dr Kais Hassan, who performed the autopsy at Iraq's Forensic Medical Institute.
Army spokesman Grey said the Pentagon lists al-Izmerly's death in an "undetermined cause" category because the body was released before army investigators learned of the case, and no US autopsy was performed.
There have been other cases in which the US military attributed to natural causes detainee deaths later found to have been caused by brutal American treatment.
"I had suspicions that this person had actually been beaten to death in the prison"
Iraq Survey Group
Charles Duelfer, the CIA special adviser who led the arms-hunting Iraq Survey Group, didn't respond to queries about what he knew of al-Izmerly's death.
But Barton, one of his former subordinates, has spoken out.
The Australian microbiologist says he was told in February 2004, while working with Duelfer's group in Baghdad, that al-Izmerly died of a brain tumour. But "I had suspicions that this person had actually been beaten to death in the prison", Barton said.
Al-Izmerly figures prominently in Duelfer's final report of 6 October as a "mentor" to Iraqi chemists trying to make poison gas for military use in the 1970s, as leader of the effort to produce mustard gas, and in the 1980s as chief of an Iraq Intelligence Service chemical section.