Six other prisoners were injured in the Monday riot that broke out shortly after noon at the Camp Bucca Theatre Internment Facility near Umm Qasr in southern Iraq.
The US command statement said the riot occurred after a routine search for contraband in one of the camp's 10 compounds, and "resulted from both the use of force to control the situation and from violence by other detainees within the camp during the riot".
"The cause of the riot and use of lethal force is currently under investigation by the chain of command and the US army's Criminal Investigations Division, which is standard procedure whenever a detainee death occurs," the statement added.
Riot quickly spread
"The riot quickly spread to three additional compounds, with detainees throwing rocks and fashioning weapons from materials inside their living areas." the statement said. "Guards attempted to calm the increasingly volatile situation using verbal warnings and, when that failed, by use of non-lethal force.
"After about 45 minutes of escalating danger, lethal force was used to quell the violence," the statement added.
The command said names of those who died are being provided to the Iraqi government and the International Committee of the Red Cross.
The US military says it holds an estimated 7000 security detainees, with 4700 of those held at Camp Bucca and 2300 at Abu Ghraib, west of Baghdad.