The interior of the FBI building is restricted to a very select group of people. The security to enter the building is extremely intensive. Watching people go in and out of the building it was clear the different levels of clearance.
Walking around the building I tried to go inside. Immediately upon entering a few people asked me who I was or what I was doing. I asked if I could visit or take at tour of any part of the interior and they told me that tours for the public were by appointment only. Shortly after I was asked to leave.
After doing more research into how tours are scheduled I came to the conclusion that the process is similar to that of White House tours. One has to schedule a tour through his or her congressman and it must be a large group.
Further reading into how to schedule a tour can be found here.