The Pentagon said about 108 detainees have died in US custody in Iraq and Afghanistan since 2002, not counting those killed in insurgent mortar attacks on detention facilities.
A report by the New York-based group Human Rights First, made available on Wednesday, also faulted "scattershot" military record-keeping on prisoner deaths and failures by some commanders to report deaths in a timely way.
"We found across the board amazing flaws and failures in the way the investigations were conducted in case after case," said Deborah Pearlstein of Human Rights First. The shortcomings undermined chances for convicting US personnel who may have broken the law, Pearlstein said.
Leuitenant Colonel Mark Ballesteros, a Pentagon spokesman on detainee matters, said, "These investigations cannot be compared to criminal investigations conducted in a typical American city.
"It is very important to understand that the majority of these death investigations are being conducted in very austere and dangerous environments, Ballesteros continued"
"We found across the board amazing flaws and failures in the way the investigations were conducted in case after case"
Human Rights First
Human rights activists and others have criticised the United States over its treatment of prisoners in Iraq, Afghanistan and the US naval base at Guantanamo Bay, Cuba.
The United States faced international condemnation after photos surfaced in April 2004 of US forces abusing and sexually humiliating detainees at Iraq's Abu Ghraib prison.
The Human Rights First report said that in many cases Army investigators did not interview key witnesses to a death or events leading to it, and in some cases autopsies were not conducted and bodies treated carelessly.
It said the military in some cases launched separate criminal and administrative investigations into a detainee's death and that those inquiries sometimes interfered with one another, compromising chances for prosecution.
The group said the report was based on an analysis of Army Criminal Investigation Command documents, press accounts and its own interviews.
US soldiers who may have acted
unlawfully are hard to prosecute
The report cited the case of Hadi Abdul Hussain Hasson al-Zubaidy, saying information from the military indicated he died at Camp Bucca in Iraq between April and September 2003. It said military investigators learned of the death about a year later, and the case was closed without a cause of death being determined.
It also noted the case of Iraqi detainee Obeed Hethere Radad, shot at a US detention facility in Tikrit in September 2003. His death was not reported for four days and the military failed to collect either the gun or bullet that killed him and conducted no autopsy.
The commander of the US soldier accused in the shooting approved his discharge from the military a week before criminal investigators found probable cause to charge him with murder, the report said.
Careless treatment of evidence - in this case body parts - weakened the prosecution case brought against Marines in the 2003 asphyxiation death of 52-year-old Iraqi Nagem Sadoon Hatab at a detention facility near Nassiriya, the report stated.
A bag containing his internal organs was left on an airport tarmac and exploded in the Iraqi heat, his rib cage and larynx were found in medical labs on different continents, and a fractured throat bone was never found, the report stated.
The report cited the death of 63-year-old Nasef Ibrahim, whose death at Abu Ghraib in January 2004 was listed as a heart problem. A later Army review found the original investigators had not interviewed key witnesses or the man's son, who had been jailed with him.
The son said US captors had beaten him and his father, doused them repeatedly with cold water, put them in stress positions, deprived them of food and menaced them with dogs.