Suppose it is known that I killed someone (for their skin to make a nice coat with, and they weren’t cool with the deal… i.e. this is murder) but I could conceivably have done it in one of many countries*. Where am I put on trial?
*I have an implausible hypothetical to explain this but no point in typing it out; I emphasise however that it isn’t something like committing the murder on an aircraft or ship.
In many countries, courts have juridiction if the victim or the criminal is one of their citizens, regardless of the country where the crime took place. If I (french citizen) murder a Mexican citizen, I could be tried in either country, for instance, even if nobody knows where I did that.
WAG: You’d be charged by whichever jurisdiction you were found with the skin, but not for murder (possibly for tampering with human remains?). Then I imagine whichever jurisdiction charged you would look for where the other crime was committed.
Maybe ignore the skin part… I was just trying to emphasise it was murder rather than justifiable homicide, assisted suicide, whatever…
Let’s make it specific: suppose you and a companion rowed a boat to this spot where France, Germany and Switzerland meet, in the middle of the Rhine. There you killed your companion, with witnesses agreeing that you killed him/her, but not clear on exactly where you were located at the time.
I suspect that the answer is that you could be tried for murder in any of the three countries.
Let me attempt to rephrase what I see as the question and give it a more specific bent as an example.
The evidence available shows the following:
Joe, a citizen of the UK, went on a vacation to the European continent, and planned to pass through Belgium, France, and Germany before returning home. No trace of his crossing either sub-national or international boundaries are tracked along his planned iternary (e.g. no passport checks/stamps/entry records are performed at the border crossings in question, so there would be no evidence whether or not he crossed, and when.). Joe turns up missing and his corpse is found stuffed in the trunk of his car which was found abandoned in Italy.
Mary, an American who had a grudge against Joe, is questioned and confesses to murdering Joe (or the evidence that she did so is overwheming), but when asked the jurisdiction in which she murdered him, refuses to testify (assume that she cannot be compelled to do so).
What now? Does she walk on the murder charge since nobody knows where the murder took place?
Since the body was found in Italy, it would be the local prosecutor’s office in Italy which would investigate the crime at first. Given the confession, I suspect that the location of the body would be a strong enough tie to Italy, and she could be prosecuted there.
There is generally a law under which someone can be charged, even if not the ‘core’ crime that is the reason for the law. For example, “entering the country as a murderer bent on escaping justice where the murder was committed” would fall under a “fugitive from justice” head in codifications of crimes.
It’s instructive to see how the U.S. criminalized “underage sex tourism” – it is not, AFAIK, a crime under U.S. law to have consensual sexual relations with an underage partner in a place U.S. jurisdiction does not extend to. But it is a crime to leave the country with the intent of having such relations.
(voice of) General Disarray: Simpsons already did it! 
(Sideshow Bob takes Bart to five corners (where five states meet), where Bob will be in one state and firing a gun [which is not illegal], the bullet with pass through another state, killing bart in a third state, while his body would fall into the fourth state. I can’t remember what the fifth state was for; maybe Bob planned to escape there, or maybe it’s just because five corners is already a Simpsons’ gag.)
Laws vary, of course, but as a general rule, you can assume that an offense can be charged in any jurisdiction where an element of the offense occurred. If someone is kidnapped in Texas and murdered in Mexico, Texas can charge the offender with both the kidnapping and the capital murder, since the conduct that elevated the offense from murder to capital murder (kidnapping) occurred in Texas.
Also, some jurisdictions have codified a legal presumption that, if a body is found in that jurisdiction, the death occurred in that jurisdiction. Going again with my state of Texas, it’s in the Penal Code, section 1.04(b), in part: “If the body of a criminal homicide victim is found in this state, it is presumed that the death occurred in this state.” Presumptions, of course, can be rebutted, but if there’s enough evidence to rebut the presumption, there’s enough evidence to charge the person elsewhere.
So they charge you in Italy, where the corpse was found, and also based on a partial confession. What’s the defense? “Prove it happened here?” Well, it happened, and the body’s here, and unless you can produce evidence that the car crossed over the border before the death, then it’s logical to assume the crime happened here too.
I assume the argument is not to “prove it happened here”, but the defense should also “prove” it happened somewhere else to cast reasonable doubt on the “it happened in Italy” theory. (I.e. body time of death vs. location of car). “ha ha you can’t prove it exactly” is not a great defense, as Scott Petersen found out.
I assume this is analogous to two people walk away with the victim who is later found shot. Both people claim the other person did it. What happens? Logically, the police charge one and use the other as a witness. (or charge both and offer one a deal)
There are a lot of people in jail for crimes they did not commit based on much more circumstantial evidence. The idea that the courts will be scrupulous about details like “where did it happen” when a crime OBVIOUSLY happened is a little naive. There have been cases, for example, where the person is charged with murder even though a body is never found.
It seems the best place to commit a murder that confuses jurisdiction is the International Space Station.
So if you had sex with someone claiming to be over 18, you might actually walk because you could prove with witnesses that you asked them how old they were and to see an ID, and they provided a fake ID and said 19 [but were 16 for sake of argument] ?
Obviously this would not work if you were in a brothel notorius for/claiming to be staffed by 12 year old virgins…
Surprisingly, maybe not - a U.S. astronaut is a federal employee engaged in official duties, so killing one on the space station would be a federal crime, even extraterritorially. Other nations probably have similar provisions.
As my old conflict tutor used to say “there is always jurisdiction”!!!
Many common law jurisdictions contain provisions whereby a murder overseas by one of their citizens is triable in local courts.
This is actually a fairly simple issue, juridiction headaches are like the old conflict question “justin a half Canadian half Sri Lankan man citizen of Holland, ordinarily resident in South Africa, marries Joan, a German-Israeli citizen in Latvia and purchases property in Pakistan…” at which point we usually fell asleep.
What happens if I murder someone on Sealand?
Many indictments are full of “on or about the time of” and “in or around the point of”… Especially for example really old child molestation cases. Failure to nail down the exact date is not considered a fatal flaw in the charge. Most likely, failure to nail down the exact location is also not going to make the case flawed.
Consider the similar case of a murder committed on a train or bus and not noticed until the it crossed state lines. (Murder on the Orient Express, anyone?) If the indictment jumps to a conclusion, you can try to prove it is incorrect - but any proof you supply is likely to be fodder for a similar charge in the other jurisdiction.
Simply arguing “it’s entirely possible it happened elsewhere” is tantamount to an admission “it’s also possible it happened here”. I think the court would be more worried about the other proof of who is the guilty party “beyond reasonable doubt” than jurisdiction beyond reasonable doubt.
Worst case, the defense shows the car definitely did not enter Italy until after the victim was dead. However, that just proves it drove in from France, and so the French would get the evidence and the jurisdiction barring evidence to the contrary. I’m sure the Italians have “possession of a dead body” and “concealing evidence” crimes to keep the pot stirred.
(BTW, keep in mind most of Europe except the UK has the Napoleonic code; the accused must testify; silence or failure to adequately explain the situation can be treated as evidence of guilt; the only proviso being that the accused cannot be charged with perjury over what they say, since it is assumed an accused will say anything to get away with a crime.)