Police found the body of her mother, Tina Smith, 41, at their home, the victim of a possible murder.
Authorities are looking for Smith's missing silver Dodge Neon, possibly driven by Jeffrey Scott Easley, 32.
The child's father is a police officer who works in South Boston, a nearby community. Police said he is cooperating with authorities in the abduction investigation.
Brittany has been missing since Dec. 2. Police believe she is in extreme danger.
Brittany is about 5 feet tall and weighs 100 pounds.
Roanoke County police are asking for anyone with information to call them at 540-777-8641 or Virginia State police at 800-822-4453.