The American Civil Liberties Union has said it identified 25 to 30 cases of "unjustified homicide" in US-run prisons in Iraq, Afghanistan and Guantanamo Bay.

After filing a Freedom of Information request in 2009, the civil rights group last week obtained 2,624 pages of documents from the US military detailing investigations into 190 deaths in custody at prisons in Afghanistan and Iraq, as well as the detention center in Guantanamo Bay.

The Defense Department says many of those deaths were due to illness, natural causes or inmate-on-inmate violence, but the ACLU alleges it has identified more than two dozen deaths it sees as being unjustified.

"So far, the documents released by the government raise more questions than they answer, but they do confirm one troubling fact: that no senior officials have been held to account for the widespread abuse of detainees," the ACLU said in a statement, as quoted at CNN. "Without real accountability for these abuses, we risk inviting more abuse in the future."

The ACLU noted that heart problems accounted for more than 25 percent of deaths, an unusually high number that "could potentially raise serious questions about the conditions of confinement or interrogation of the detainees."

The civil rights group says that while many of the deaths were previously known, some had never been revealed publicly. CNN reports:

In one such case, a detainee was killed by an unnamed sergeant who walked into a room where the detainee was lying wounded "and assaulted him ... then shot him twice thus killing him," one of the investigating documents says. The sergeant than instructed the other soldiers present to lie about the incident. Later, the document says an unnamed corporal then shot the deceased detainee in the head after finding his corpse.

In another example, documents note a soldier "committed the offense of murder when he shot and killed an unarmed Afghan male." But, according to the ACLU, the individual was found not guilty of murder by general court-martial.

The Defense Department defended its record, saying that the very existence of the thousands of pages of documents shows it takes in-custody deaths seriously. Army spokesman Lt. Col. David H. Patterson said that of the 190 deaths, 43 had US soldiers or personnel as suspects, resulting in 13 findings of probable cause for murder, and 19 separate convictions.

The ACLU's document release came the same week as the Obama administration let it be known it plans to resume the use of military commissions to try terrorism suspects at Guantanamo Bay, a move the ACLU described "strik[ing] a major blow to any efforts to restore the rule of law."

"The decision to proceed with commissions ... raises serious questions about whether commissions are being used as a forum to hide the use of torture and base convictions on evidence that would be too untrustworthy to be admitted in any real court," said Hina Shamsi, director of the ACLU's National Security Project.

"Unlike federal courts, which have well-established rules of procedure and evidence, the military commissions rules do not comply with US and international law," Shamsi added.