The shooting is believed to have occurred at the Carl R Darnall Army Medical Center, located at the Fort Hood military base. A shelter in place order has been issued for the civilians and soldiers.
The gunman is still on the loose and believed to be in the Medical Brigade Building, television news channel KCEN reported.
Nearby Central Texas College in Killeen immediately evacuated all students and staff after the shooting incident. The classes for the afternoon and evening have been cancelled.
The shooting comes one day after the US Marine Corps confirmed an FBI search is underway for a former Army recruit, identified as Muhammed Abdullah Hassan or "Booker", who was plotting a "Fort Hood-inspired jihad against US soldiers.