What a sad situation! It sounds like you have concerns about the foster home, and it's hard to tell what all is going on there. I can see how something like this could happen even in a very responsible foster home. If the dog has enough drive to go through a window, there's a lot of cat access out there. So it's hard to know if the foster home screening was even a risk factor in this particular case.
In trying to figure out what happened, the approach I try to take in sensitive situations is to treat everyone involved with an abundance of compassion and respect, assume that they were trying to do their best, and recognize that they are likely feeling vulnerable. You may find the foster more open to a questions if your conversation is positioned around a mutual understanding that what happened was heart-breaking and stressful for everyone concerned, that you're wanting to check in on her and see how she is doing, and that as part of consistently trying to improve your organization's programs, you're trying to understand what happened best so you can see if there are any opportunities to put measures into place that might help prevent future occurrences. From that perspective, she may be more open to talking you through her understanding of what happened. It may be that this was just a tragic accident and there isn't anything that could be done, or it may be that you'll find something that will be helpful moving forward.
In any case, I support you continuing to let things go. It isn't sustainable for you to try to manage everything yourself!
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