I once had pet mice. 2 of them. 1 died so I got 2 more to keep mine (the other was my sister's) company. They lived for a while then died. Mine was old by this time so I figured she'd just die, but she didn't, she kept on living, so I got another one, and they got along really well. Then one day the young new was was dead, and I think that she may have killed it.

But we'll never know. 

Maybe she killed all of them. 

It will forever be a mystery, because she finally died, we'll never get to ask her.