Comprehensive testing has confirmed the presence of the chemical weapon sarin (search) in the remains of a roadside bomb discovered this month in Baghdad, a defense official said Tuesday.

The determination, made by a laboratory in the United States that the official would not identify, verifies what earlier, less-thorough field tests had found: the bomb was made from an artillery shell designed to disperse the deadly nerve agent on the battlefield.

The origin of the shell remains unclear, and finding that out is a priority for the U.S. military, the defense official said, speaking on the condition of anonymity.

Some analysts worry the 155-millimeter artillery shell, found rigged as a bomb on May 15, may be part of a larger stockpile of Iraqi chemical weapons that insurgents can now use. But no more have turned up, and several military officials have said the shell may have been an older one that predated the 1991 Gulf War.

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It likewise is not known whether the bombers knew they had a chemical weapon. Military officials have said the shell bore no labels to indicate it was anything except a normal explosive shell, the type used to make scores of roadside bombs in Iraq.

No one was injured in the shell's initial detonation, but two American soldiers who removed the round had symptoms of low-level nerve agent exposure, officials said last week.

The shell was a binary type, which has two chambers containing relatively safe chemicals. When the round is fired from an artillery gun, its rotation mixes the chemicals to create sarin, which is supposed to disperse when the shell strikes its target.

Since it was not fired from a gun but was detonated as a bomb, the initial explosion on May 15 dispersed the precursor chemicals, apparently mixing them in only small amounts, officials said then. In battle, such shells would have to be fired in great numbers to effect a large body of troops.

Iraq's first field-test of a binary-type shell containing sarin was in 1988, U.S. defense officials have said.

Saddam's government only disclosed the testing and production after Iraqi weapons chief Lt. Gen. Hussein Kamel al-Majid (search), Saddam's son-in-law, defected in 1995. Saddam's government never declared any sarin or shells filled with sarin remained.

Saddam's alleged stockpile of weapons of mass destruction (search) was the Bush administration's chief stated reason for invading Iraq. U.S. weapons hunters have been unable to validate the prewar intelligence.

Some trace elements of mustard agent (search), an older type of chemical weapon, were detected in an artillery shell found in a Baghdad street this month, U.S. officials said previously. The shell also was believed to be from one of Saddam's old stockpiles.