USB is probably the easiest way it could've been done. Drop a USB in a parking lot, or near an employee area. What would you do if you found a USB on the ground? Plug it in and see what's on it so you can keep it or return it. Employee finds it, plugs it in, and its over before he even knows what just happened. There were rumors that that's how the US shutdown some foreign nuclear facilities. It caused part of the machines to start and stop very abruptly, eventually breaking each piece and rendering the facility worthless. (very large and expensive pieces. Im not a nuclear physicist, I don't know what they were) it wasn't traceable once in the system, not even noticeable really. It appeared to be just faulty machinery.
Anyways, I have no proof to support it, just word of mouth. Its a neat little hackers bed time story.