I filed application for US citizen on 02/06/13,I had interview on 05/22/13 and past exam, immigration officer requested additional information, it was sent by certificate mail and I got return receipt on 05/26/13. So far almost six months and nothing has received from immigration, I had an appointment to check in the window of immigration office two months ago, the answer was my case is under security check. How long do I have to wait? What should I do for the next step? It must be very appreciated for your help.