There can be far more done than just an autopsy in the second case. Is there a register who has entered and left the building? Is there camera footage showing anyone accessing the room that had no business being there? Is there anything unusual in the nurses schedules? Were all procedures followed according to the rules, especially sanitary rules?
These are all things that should be investigated. If they show no signs of irregularities then the case can be closed. If there is irregularities, then these need to be investigated further, and then the question of motive comes into play, where there is one party with a very strong motive to silence the guy.