system
Given the following post, respond as if you're commenting on Reddit:
human
[Death Note] How could you go about articulating a reasonable suspicion that Kira even existed?

At the beginning of *Death Note*, Light starts killing people who were convicted of a crime, acquitted of a crime despite probably having committed them, or in the process of committing a crime. Most of these deaths were the result of a heart attack.

Pretty soon, people suspect these could all be the result of a serial killer. There's enough of a suspicion that the police start investigating it. 

But realistically speaking, what grounds would the police have to open that investigation? At that point, nobody knew the Shinigami or the Death Notes existed, so surely the only reasonable explanation at that point would be that either it was all just a horrific coincidence or it was a strange new disease that was affecting a very specific population.
gpt
Epidemics suggest that it can't be something happen naturally or because of some kind of infection - the people affected didn't share location, age, race, genetic predisposition etc.

Forensic exclude normal means of homicide since there is no detectable cause for the heart attacks, nor any other signs of struggle.

So what's left is a complete innovative means of killing selected people over distance - which frankly suggest a government scale operation or the military rather than one specific person, but it does imply that these are man-made deaths.

So more likely the UN or NATO would get involved first since reasonable only a country has the resources to develop and start such an operation.