The tomb "once contained an Unknown from Vietnam (middle). His remains were positively identified in 1998 through DNA testing as First Lieutenant Michael Blassie, United States Air Force and were removed." (Info on Wiki and taken from the tomb website). It's now empty as a symbolic tribute to other unknown fallen troops.
I was just out there last week. If you are planning a visit, they are doing a major renovation on it. Was dissapointed we could not get very close. It is a must see if you visit Washington, D.C. Just as the Arizona memorial is if you ever go to Hawaii.
Thanks! Your answer is awaiting moderation.