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Ok, so now that that is done. I agree with the two above me and you should get your local authorities invovled. I'm not sure what the full retail is on a NII but it has to be above $500 and that makes it a viable option for more than just a complaint.
What I would do is since this is a kiosk type thing in a mall (<--key point there), I would go to the mall security and have them review the tapes. Kiosks are out in the open, so they are not covered by internal cameras and loss prevention like the Gap or Finish Line. There camera security is the same as the mall's, so since this took place in the mall's space, they would be the ones that you would want to go to. Tell them what happened and the value of the phone (to put a little weight on your issue) and see if they can pull up the footage. Tell them that you are seriously thinking about police action and this could determine whether or not you do. Just an idea.
"The work of men of genius, however wrongly directed, hardly ever fails to open the eyes of men to new possibilites."
- Mary Shelley