U.S. investigators haven't ruled out the possibility that an Iraqi artillery round containing the nerve agent sarin, which was used by insurgents last month in an ineffective homemade- bomb attack, could be part of a chemical-weapons stockpile. Most investigators believe the insurgents who put the device together didn't know the old shell they had contained sarin. But a few U.S. experts note that analyses of homemade bombs found in the same area as the sarin device indicated that insurgents usually drill out the tops of old shells and put new detonators inside. But in the case of the sarin shell device, the bombmakers didn't drill out the top of the old munition; they placed a charge next to it, which suggests to some U.S. investigators the bombmakers knew the shell contained nerve gas. What scares the pessimists is the possibility that insurgents who used the sarin shell are members of Saddam Hussein's regime who have access to a hidden cache of similar munitions. Because of the age of the recently discovered sarin shell, however, few U.S. experts believe it's a big break in the hunt for unconventional weapons.