Human Rights Watch (search) said Monday it had uncovered two more cases of prisoners dying in American custody in Afghanistan (search), and it accused the Bush administration of "dragging its feet" on investigations that could have prevented the abuse of prisoners in Iraq.

In an open letter to Secretary of Defense Donald Rumsfeld, the New York-based rights group said it had new evidence of "an alleged murder of a detainee by four U.S. military personnel" in Afghanistan in 2002. More recently, it said a man picked up on Sept. 24, 2004, died the next day at an American base, but it did not specify the cause of death.

"It's time for the United States to come clean about crimes committed by U.S. 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."

U.S. military officials in Afghanistan had no immediate comment on the alleged new cases highlighted in the letter.

Military investigators are already examining the deaths of several prisoners in U.S. custody here, but have released few of their findings.

Rights groups have been particularly critical of the slow pace of a criminal investigation into how two Afghans died at the main American base at Bagram (search), north of the capital, in December 2002.

The Army announced in October that up to 28 U.S. soldiers face possible criminal charges including involuntary manslaughter and maiming in connection with the deaths, both of which were ruled homicides.

Some of those expected to face charges are from the 519th Military Intelligence Battalion. Some members of the 519th went from Afghanistan to Iraq in 2003 and are among those accused by Army investigators of abusing Iraqi detainees in fall 2003.

In its letter to Rumsfeld, Human Rights Watch said the alleged murder happened in or before September 2002 — even earlier than the Bagram case.

The group said recently released Pentagon documents from an Army investigation at the time stated that the four soldiers "murdered" the man "after detaining him for following their movements." Human Rights Watch said the case was apparently closed and unspecified action taken against the soldiers.

It identified the man who died on 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 shot and killed by U.S. forces.

Khan died at an American military base and his family told Afghan rights investigators that the body was bruised when they retrieved it, Human Rights Watch said.

Khost Gov. Merajuddin Patan said Monday that U.S. officers told him that a man had died in their custody at about that time of a heart attack.

Human Rights Watch called for U.S. officials to explain both deaths and said they helped highlight the American government's "failure to establish accountability for abuses."

The failure to prosecute abuses in Afghanistan dating back to 2002 "spawned a culture of impunity" among some interrogators, and allowed abusive interrogation techniques to spread to Iraq. "The U.S. government is dragging its feet on these investigations," Adams said.

The group also called for the release of a review of the about 20 U.S. holding facilities at bases scattered across Afghanistan which was ordered by the top American commander here in May but remains under review by his superiors.

Lt. Gen. David Barno has said that many improvements were made to procedures for handling prisoners in the light of the cases of alleged abuse as well as the still-classified review.