"Williams said investigators initially believed the tanker was stolen for parts or to dump toxic waste illegally.
"If that were the case, we would have found the carcass" of the tanker, she said."
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This is why they are worried now. At first they thought it was an "ordinary" theft, but when they couldn't find it, even with some effort, they are concerned what it could be used for.
Well, she is a govt. employee :-)
Well, we weren't worried 7 months ago, but now that it will be just about impossible to find, we are a bit concerned.