On Sunday afternoon, before the American offensive in Kufa, General Kimmitt said it was unclear why insurgents were still attacking American soldiers. Perhaps it was because those fighters had not received word of the cease-fire, he said, or perhaps they belonged to a splinter group that did not take orders from Mr. Sadr.
http://www.nytimes.com/2004/05/31/international/middleeast/31CND-MILI.html?pagewanted=2&hp

General Kimmitt said his understanding was that the agreement for the fighters to lay down their weapons applied to both Najaf and Kufa, but he suggested that Mr. Sadr might have interpreted the agreement as applying only to Najaf. In any case, General Kimmitt said at a news conference: "Moktada's militia is a declared hostile force. Our soldiers have the obligation to take action."

Under the conventions of war, soldiers may attack combatants of a "declared hostile force" without being shot at first. General Kimmitt was in effect saying that American soldiers had the right to shoot at insurgents displaying weapons.