From the Washington Post:
Shiite Muslim militias pose the greatest threat to security in many parts of Iraq, having killed more people in recent months than the Sunni Arab-led insurgency, and will likely present the most daunting and critical challenge for Iraq’s new government, U.S. military and diplomatic officials say.
Assassinations, many carried out by Shiite gunmen against Sunni Arabs in Baghdad and elsewhere, accounted for more than four times as many deaths in March as bombings and other mass-casualty attacks, according to military data. And most officials agree that only a small percentage of shooting deaths are ever reported.
Here.