Seems to me, that the police on the US side of the border were guilty of a procedural error only, really. That being said, I don't see how they would be directly, or indirectly responsible for the womans death. That responsibilty lies with the criminal.
Canada and the US share the longest undefended border in the world.
Police forces on both sides are generally underfunded, and short on manpower. They are severely overworked.
It would be nice if some professional courtesies could be in play here. They oughta have a buffer zone wherein both police forces could cross border, provided that the other force is notified that its about to happen (or has in fact just happened).
From what I have read so far, this whole thing happened pretty fast. Had the Canadian police been informed, it is pretty doubtful they would have been able to be "on scene" immmediately, as these two US officers were already.
So, did the two US officers mess up?
Yep, but only (IMHO) because there doesn't seem to be any legal professional courtesies allowed, and they did in fact enter Canada illegally.
Did they cause this womens death, or contribute to it?
Hardly.
RTR