If they cannot possibly be used in the manufacture of explosives, why is the Joint Terrorism Task Force investigating it, as opposed to treating it as a simple theft, to be investigated only by the local authorities?
Answer: because they can be used to make explosives, although the FBI lies about this, as with so many other things. When will it become a crime for them to lie to us, just as it's already a crime for us to lie to them?
< /paranoia > You forgot to close your tag.
Perhaps because the folks who stole the material, didn't know it couldn't be used for explosives.
Because the ragheads stole the wrong stuff?
I had the same initial question tha you did, but after a bit of thought figured the reason for FBI involvement is that the theft might be a sort of "honey pot" for mad bombers, rather than a simple theft. If the thieves think it can make explosives, that's a good reason for the FBI to look for them.