In order to understand the seriousness of the chemical weapons threat against U.S. forces in Iraq, it is necessary to determine who was behind the May 17 attack against a U.S. convoy. An artillery shell containing sarin, a combat chemical agent, was used to make an improvised explosive device (IED). Information coming from the U.S. military and foreign sources, as well as knowledge of how sarin-charged shells operate, suggests Iraqi nationalist guerrillas — likely former members of the Iraqi military — were behind this attack. We will examine evidence that could lead to determining the identity of the attackers. The U.S. Command said three important things in this regard: that the amount of sarin in the IED was small, that the sarin-charged part of the IED exploded only when the U.S. sappers tried to defuse it and that the sappers were only lightly exposed.