At least 108 people have died in US custody in Iraq and Afghanistan, according to figures compiled by the Associated Press news agency.
Human rights groups say top US officials should be held accountable
Most deaths were violent and some 25% are being investigated as possible abuse by US personnel, the agency said.
The death toll - far higher than previously thought - was based on information the agency obtained from the US army, navy and other officials.
The Pentagon said it was important to bear in mind the context of each death.
Some had died of natural causes, others had been victims of insurgent attacks on US detention facilities and some killed in violent prison uprisings, a spokesman told the BBC News website.
"That said, each of these deaths is investigated to determine the circumstances and whether there is any accountability," he added.
More than 60,000 people have been taken prisoner since the US-led wars in Afghanistan, in November 2001, and Iraq, in March 2003. Most have been freed.
The AP found that of the 108 deaths in US custody:
- At least 26 have been investigated as criminal homicide involving the abuse of prisoners
- At least 29 are attributed to suspected natural causes or accidents
- Twenty-two are blamed on an insurgent mortar attack on Iraq's Abu Ghraib prison in April 2004
- At least 20 are attributed to "justifiable homicide", where investigations found US troops used deadly force appropriately - primarily against rioting, escaping or threatening prisoners.
Last week, a Pentagon report to Congress into prisoner abuse in Iraq and Afghanistan cited only six prisoner deaths in what it called "closed, substantiated abuse cases" as of last September.
The report's author, Vice Adm Albert Church, blamed the abuse on a breakdown of discipline, but did not directly criticise any high-level officials.
The American Civil Liberties Union said it was "unacceptable" that no-one at the highest levels of government had been held accountable for the abuses.
"Despite the military's own reports of deaths and abuses of detainees in US custody, it is astonishing that our government can still pretend that what is happening is the work of a few rogue soldiers," said ACLU Executive Director Anthony Romero.