U.S. guards opened fire Monday on prisoners during a riot at the main detention facility for security detainees, killing four of them, the U.S. command said. Six other prisoners were injured.

The riot broke out shortly after noon at the Camp Bucca internment facility near Umm Qasr (search) in southern Iraq after a routine search for contraband in one of the camp's 10 compounds, the command said in a statement.

"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 statement said the riot "resulted from both the use of force to control the situation and from violence by other detainees within the camp during the riot."

The command said names of those who died are being provided to the Iraqi government and the International Committee of the Red Cross (search).

"The cause of the riot and use of lethal force is currently under investigation by the chain of command and the U.S. Army's Criminal Investigations Division, which is standard procedure whenever a detainee death occurs," the statement added.