Los Angeles County Sheriff's Spokesman Deputy Jeff Gordon said the girl's uncle, 25-year-old Julian Carter, was arrested in connection with the death. Neighbors reported Carter had a developing drug problem.
Deputies searched the girl's home Sunday morning after her mother reported her missing.
During a standard check of the house to make sure she wasn't hiding anywhere, the girl's body was found in a closet.
Homicide detectives are investigating.
The Associated Press contributed to this story.