From the NY Times:

"United States soldiers opened fire on Iraqis at an antiwar demonstration here, and according to local hospital officials killed 15 people and wounded about 75. An American officer said today that the troops opened fire after being "intentionally" shot at by some of the protesters, a claim disputed by residents."

I've seen claims that they fired on protestors after they threw rocks at our troops. Looks like this is something we'll really never know the answer to.