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a U.S. convoy in Iraq found an artillery shell believed to have the makings of sarin -- a deadly nerve gas used in chemical weapons, the coalition said Monday.

A senior defense official said a preliminary field test had been completed on the shell, which will undergo further testing. Field tests sometimes yield false positives.

Kimmitt said the shell contained two chemicals which, when mixed during the flight of an artillery shell, formed the nerve agent.

He said the shell had been rigged as a makeshift bomb that resulted in a small dispersal of the agent when it exploded before an ordnance team could disarm it.

U.S. intelligence officials in Washington said the shell was discovered Saturday near the Baghdad International Airport.

"The area that was affected was very minor," Kimmitt said. "There's no need for any further decontamination. The [ordnance team] people who went up there showed some minor traces of exposure, but it was so minor the doctors already have these people released."

Kimmitt said the artillery round was of an old style that Saddam Hussein's regime had declared it no longer possessed after the Persian Gulf War.

Kimmitt said it appeared that whoever set up the roadside bomb was unaware that it contained the chemicals.

"It was a weapon we believed was stocked from the ex-regime time," Kimmitt said. "It had been thought to be an ordinary artillery shell, set up like an IED [improvised explosive device]. When it exploded, it indicated that it had some sarin in it."

The general said the Iraqi Survey Group, headed by Charles Duelfer, would determine if the shell's discovery indicated Saddam possessed chemical weapons before the U.S. invasion last year. Officials in Washington said another shell -- this one containing mustard gas -- was found 10 days ago in Iraq.

No other evidence of possible chemical weapons has been found in Iraq. The Bush administration cited weapons of mass destruction as a key reason for its invasion.