well, you can't overplay it - then you open yourself up to the media saying you've hyped it.
But say SOMETHING - and not just from Kimmitt. An appropriate statement would be:
"today, a nerve gas agent was part of an explosive device used by insurgents to attack US troops. The US military is investigating, and using all efforts to determine the source of this material and the storage location where it came from".
Put the premise on the table that the material was used, and that it must have come from someplace inside the country - where there is one, there are others.
Once the news cycle passes, the media gets to define this story on their terms (pesticide, etc) - and after that, the truth no longer matters.
I think you are correct that the admin needs to promote this a bit, without claiming too much. It is highly unlikely that of the hundreds of thousands of tons of munitions still in Iraq that this one shell is the only chemical weapon in the entire country. The odds are that there are dozens or even hundreds more. Maybe even thousands.