I believe Security check, Background check and Domestic investigation are all the same terms and the case has been moved to the Review Department. You are screwed once you end up in this black hole and can do nothing about it. It also means that you are the lucky choosen one
BTW I am also stuck in the same. Good luck!
My 140 have been pending since the AD of 12/29/2006 and just got it approved on July 23. I called TSC on July 19 to check the status and inquired about the background check. The CSR mentioned that NO background check was requested for my 140. Being from a 'Muslim' country i was expecting delay with long background check, but it didn't happen. So i wonder, how they pick cases to send for security check, apparently, not all the 140's as my middle name is one of very common Muslim name. Is it luck? or is it USCIS's own unique ways?