Rat killing another rat

Question
Hi there! My poor hysterical sister in law called me this morning. She and her son woke up to a grewsome sight. One of their rats (one of 3 females)was dead in the cage and was opened in the abdominal area. I don't want to get grewsome in my description but you can imagine what they saw.
I have had pet rats for years and I have never heard of a rat mutilating another rat in this way. They have been cage mates for at least 6 months and I'm wondering if this is normal behavior. She is concerned that the rats that remain will now be aggressive towards each other or her son.

Please let me know if this is something that is common. As I said before I've never heard of a rat doing this to a cage mate before. She is so upset she wants to drop the rats off a the pet store but I would hate for her to do that.

Please advise. I appreciate your feedback. You have helped me with problems with my babies in the past and I appreciate what you do.
Thank you!

Answer
Chances are the rat died from something else....heart related etc..but I doubt they killed her.  

Truth is, one way rats bury their dead is to consume them.  The first place they go is the abdomal area. That sounds gross, yes....but this is a way they do it in the wild to keep their beloved litter mate from being dragged off by predators/buzzards etc....which can also expose their nest.