I really do wish we had the proper facilities to remove dogs in cases like this and conduct behavioural assessments on them.
We do have a small handful of experts in the UKwho could do this, and there is a chap in the US who does this and provides some useful and interesting data - for example, he has determined that a dog has been used to cover up child abuse... and also that a dog bit and killed someone whilst protecting their owner following extreme provocation (just two examples of many, I can't discuss a lot of them as frankly its too graphic!)..
But this would require purpose built kennel facilities that we just do not have - you cannot run a sensible, useful and informative behaviour assessment on a dog in a typical boarding kennel set up, and you cannot house a dog that has killed, in a typical boarding kennel set up, and the only housing available to the police is... boarding kennels (who bid for the LA contracts to house seized dogs).
A postmortem can tell you very very little about the causes of behaviour, if theres no brain tumour and no obvious physical injury such as severe joint issues or spine problems... lets say serious trapped nerves and muscle damage.. thats just not going to show up - nor of course can you behaviourally assess a dead dog.
I am not surprised to learn the baby was actually in the arms of the mother ... thats actually a pretty major trigger for many dogs, something small and prey-like (small, squeaky, noisey, moving) held up above a dogs head...
One of the things I have all clients who are planning on having a baby do, is to teach their dog to ignore them carrying something, teach them not to jump up and grab that 'something' and get used to people doing tasks and dog related chores one handed whilst carring 'something' (and we move from a totally inert object like a bag of flour, to something more interesting like a toy doll, and then add in the sounds a baby might make via smart phone apps)... because this is SUCH a massive trigger for many dogs.
This mother almost certainly didn't know that as this wasn't her dog, and its likely the owner didn't know this as she'd never seen the dog in that context before.
Tragic, and worse, probably avoidable if only proper preparation had taken place :(