KABUL, Afghanistan -- Human Rights Watch said yesterday that it has discovered two more US detainee deaths in Afghanistan, including an apparent murder more than two years ago, and said slow-paced investigations had ''spawned a culture of impunity" that may have fueled prisoner abuse in Iraq.
In an open letter to Secretary of Defense Donald H. Rumsfeld, the New York-based group presented evidence of ''an alleged murder of a detainee by four US military personnel" in Afghanistan in 2002.
It also said a man picked up on Sept. 24 died the next day at a US base, but did not specify the cause of death.
''It's time for the United States to come clean about crimes committed by US forces in Afghanistan," said Brad Adams, the group's Asia division director. ''The United States has to get serious about prosecuting people implicated in prisoner deaths and mistreatment."
The failure to prosecute abuses in Afghanistan ''spawned a culture of impunity" among some interrogators and allowed abusive interrogation techniques to spread to Iraq, Adams said. ''The US government is dragging its feet on these investigations."
US military officials in Afghanistan and Washington declined to specifically comment on the alleged new cases. But a Pentagon spokesman, Lieutenant Colonel John Skinner, said commanders went to ''enormous lengths to investigate any credible allegations of detainee abuse."
''We've looked at detention operations from A to Z," he said.
If confirmed, the latest deaths would bring to at least seven the number of Afghans who have died in US custody. Officials have previously acknowledged investigations into five deaths, but have released few of their findings.
Rights groups have been particularly critical of the slow pace of a criminal probe into how two Afghans died at the main American base at Bagram, north of the capital, in December 2002.
The Army announced in October that up to 28 US soldiers faced possible criminal charges, including involuntary manslaughter and maiming in those deaths, both of which were ruled homicides.
Some of those expected to face charges are from the 519th Military Intelligence Battalion. Some soldiers of the 519th went from Afghanistan to Iraq last year and are among those accused by Army investigators of abusing Iraqi detainees.
In its letter to Rumsfeld, Human Rights Watch said the alleged murder happened even earlier than the Bagram case, sometime before or during September 2002.
The group said recently released Pentagon documents from an Army investigation at the time stated that four soldiers ''murdered" an Afghan ''after detaining him for following their movements." The letter said the case was apparently closed and unspecified action taken against the soldiers.
It identified the man who died this Sept. 25 as Sher Mohammed Khan and said he was arrested during a raid on his family's home near the eastern city of Khost in which his brother was fatally shot by US forces.
Khan died the next day at an American military base and his family told Afghan rights investigators the body was bruised when they retrieved it, Human Rights Watch said.
Khost Governor Merajuddin Patan said yesterday that US officers had informed him of a man dying in their custody around that time of a heart attack. He said he had no further details.
Human Rights Watch called for US officials to explain both deaths and said they highlighted the American government's ''failure to establish accountability for abuses."
The group also called for the release of a review of US holding facilities at bases scattered across Afghanistan. The review was ordered by the top American commander here in May but remains under review by his superiors.
Lieutenant General David Barno has said many improvements were made to procedures for handling detainees in light of the cases of alleged abuse as well as the still-classified review.