What would the Muggle autopsy of a Killing Curse victim reveal?
Wouldn't it sound unusual to Muggles that a person died because of absolutely nothing?
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The accepted answer is correct, but I feel this question is lacking in some book quotes. This, then, is the actual quote from The Goblet of Fire:
Then, just when things were looking very serious for Frank, the report on the Riddles' bodies came back and changed everything.
The police had never read an odder report. A team of doctors had examined the bodies, and had concluded that none of the Riddles had been poisoned, stabbed, shot, strangled, suffocated or (as far as they could tell) harmed at all. In fact, the report continued, in a tone of unmistakable bewilderment, the Riddles all appeared to be in perfect health - apart from the fact that they were all dead. The doctors did note (as though determined to find something wrong with the bodies) that each of the Riddles had a look of terror upon his or her face - but as the frustrated police said, whoever heard of three people being frightened to death?
Harry Potter and the Goblet of Fire - p.9 - Bloomsbury - Chapter 1, The Riddle House
This, though, was the 1940s and medical science has moved on a bit since then, so perhaps we can find a slightly stronger answer?
Not much stronger, but Dumbledore has this to say about the perplexing deaths of the Riddles:
'The Muggle authorities were perplexed. As far as I am aware, they do not know to this day how the Riddles died, for the Avada Kedavra Curse does not usually leave any sign of damage ... the exception sits before me,' Dumbledore added, with a nod to Harry's scar.
Harry Potter and the Half-Blood Prince - p.343 - Bloomsbury - Chapter 17, A Sluggish Memory
Now if we assume Dumbledore is entirely right about this, and it leaves no trace - internal or external - then the autopsy would be completely useless and would reveal nothing.
However, Muggle medical science seems to be a little more advanced in terms of our fundamental understanding of the inner workings of the human body, whereas wizard medicine doesn't seem to rely on deep biological understanding, but on ... well ... magic. Witness the fact that Harry still wears glasses. Now vision, as discussed in the linked question, is able to be corrected in many cases by changing the shape of the eye, and although this doesn't work across the board, it seems like some of the many witches and wizards who wear glasses could be helped by such modifications. I think it inconceivable that wizards would be incapable of making these modifications - either with magic or without - which seems to suggest that they do not understand. Which fits with the general view we get of wizard science (or lack of science).
Having said that, wizards do have an entrail expelling curse, which was developed by a Healer, which seems to suggest some effort to understand anatomy. Or at least, the Healer was memorialised for it, so even if he didn't invent the Curse for this purpose, it seems to have been co-opted for it.
It seems likely, though, that these efforts were not enough to know about internal signs of damage that the Killing Curse might leave. So I think there is some possibility that modern medicine might be able to detect some mechanism of death, whereas wizards would be unable to find any sign of damage, but I think this is unlikely. The strong sense I get is that the author, by these quotes, wants to get us to view the Killing Curse as leaving no mark or trace.
One thing we might also consider is that it is not clear how the curse actually kills: link. Reading this link might lead you to draw your own conclusions about what the Curse does to kill you. It doesn't seem to kill you by trauma, but it certainly renders your body uninhabitable by your soul and causes you to pass on. So it is either some kind of soul-remover, or it must break the body. And if it breaks the body, it might be possible to see the chain of events that did this, it might be possible to discover something in an autopsy.
Something to think about in this regard. If the Killing Curse is purely a soul remover, it seems odd that in the battle between Dumbledore and Riddle, it should have caused a desk to burst into flame:
He sent another killing curse at Dumbledore but it missed, instead hitting the security guard's desk, which burst into flame.
Harry Potter and the Order of the Phoenix - pp.717-8 - Bloomsbury - Chapter 36, The Only One He Ever Feared
Of course (without really knowing anything about the biology of death by natural causes), you might see nothing more or less than (the results of) a seemingly unprompted shutting down of vital functions? Or perhaps cessation of all metabolic activity, or something like that?
Whatever the cause, when you die, you obviously stop breathing, your heart stops beating, your brain stops working, your organs stop doing anything and you start to decompose. Now obviously that would all have been noted in the case of the 'completely unharmed' Riddles. Clearly, in an autopsy you're looking for whatever would have prompted that. You might not see anything, it might be that the Killing Curse simply shuts everything down all at once by magic without leaving any trace other than the fact that everything's been shut down.
To me, this seems the most likely case. There seem to be no grounds to imagine the Killing Curse attacks something specifically, disables something specifically, or otherwise causes discernible damage (apart from the death bit). Death, after all, is instant.
From Harry Potter Wiki:
There are no secondary effects; the victim simply "drops dead" for no biological reason. Victims of the Killing Curse that have been examined with Muggle methods are known to show no visible signs of trauma. Though Muggle methods are unable to identify the means of death, Ministry of Magic specialists are familiar with the markings, or lack of them. The killing curse is known to leave no identifiable marks or any signs of the reason for death. Victims appear to "drop dead", or to appear "perfectly healthy apart from the fact they were all dead."
As far as Muggle reaction, I would bet that medical examiners and coroners would list the cause of death as "Natural Causes", so the public as a whole would probably just view it as just another death.