Police hope viewers of a security camera video showing a woman beating a girl will help them find the pair.
Mishawaka, Indiana's police chief appeared on news shows Friday morning. He said he believes the woman is from an area near Fort Worth, Texas and has a criminal record.
The chief said investigators have spoken with the woman's family, but they aren't cooperating.
A prosecutor told CBS's "The Early Show" that the suspect's sister is in custody. She was also seen on the video. The prosecutor said the sister was detained because she gave deceptive answers to police questions.
The tape shows the woman placing the child -- who's believed to be about four years old -- in the back seat of an SUV at a store parking lot -- and then striking her repeatedly.
The woman is thought to be the child's mother. She's been identified as Madelyne Gorman or Madelyne Toogood.