Police say there was some kind of disturbance at the office before the blast. An officer saw someone get out of a sport utility vehicle and run behind the building. Then something exploded.
Firefighters are still battling a small blaze that flared up after in initial blast.
The suspect's body is still in the building in Dalton, about 25 miles southeast of Chattanooga, Tenn.
Two of the injured were treated at a hospital and released, one was admitted and a fourth was taken to a burn center.