Agression after breeding

[bellgl]

I have a Chocolate Lab (Chloe) that is 5 years old. I bread her in February and kept one of the pups (also a female- Riley). The Chloe grew up with a border collie, Jewell (also female now 10 yrs old) showing now signs of agression. They constantly played together and had no altercations. As the puppy has grown older she has began playing with the border collie. Chasing balls, sticks, toys etc. At one point a few months back, Jewell growled at the puppy and Chloe jumped her. My 16 year old daughter was present and was able to separate them. This happened again several weeks later and the Chloe was more agressive pinning the border collie to the ground again broken up by my daughter. Both instances occured when I was not home. I played with them several times since this putting them in the same scenario but Chloe never showed any signs of agression. I made a fatal mistake and assumed the agressive behaviour was a fluke and that Chloe would not take it any further. On Monday, we came home to find our border collie dead. I'll never know for sure, but we are certain Chloe killed her. Obviously we are devastated. But, I'm very concerned that this behaviour may occur again. Do you have any advice?

[Annie]

I am very soryy for the death of your dog. I hope someone will find some advice for you soon, but I am really sorry for your loss. It is really hard to love an animal that we don´t fully trust. You must be feeling very confused and distrustful.
Thinking of you all,
Annie