one by one they die

Not long ago, my partner's cat gave birth to four kittens. I'm not so sure about it, but they look as if they are premature kittens. The little ones are so thin and weak. She and her siblings tried to nurse the kittens from day one since the mother cat isn't giving the attention they need. We also bought cat supplies and several things we think the mother cat needs so she can nurse her kittens.

After a week, they regained strength and were able to interact to call the mother cat when they need them. However, it's just strange that one by one the kittens die. It seems like something is killing them because those that dies first are the healthiest and cutest. For now, there is only one kitten left after giving all our efforts to save them all. We are just hoping that he can survive.