From a legal prospective, a crime against humanity is defined under the Rome Statute of the International Criminal Court, which is connected with the Hague. The ICC would have jurisdiction and the hearing would be conducted under its rules and at the Hague. The crimes are:
For the purpose of this Statute, "crime against humanity" means any of the following acts when committed as part of a widespread or systematic attack directed against any civilian population, with knowledge of the attack:
- murder;
- extermination;
- enslavement;
- deportation or forcible transfer of population;
- imprisonment or other severe deprivation of physical liberty in violation of fundamental rules of international law;
- torture;
- rape, sexual slavery, enforced prostitution, forced pregnancy, enforced sterilization or any other form of sexual violence of comparable gravity;
- Persecution against any identifiable group or collectivity on political, racial national, ethnic, cultural, religious, gender;
- enforced disappearance of persons;
- crime of apartheid; or
- Other inhumane acts of a similar character intentionally causing great suffering, or serious injury to body or to mental or physical health.
Any crime against humanity would definitely be against Spanish law on an individual basis and mostly against US law (there are issues involving our interrogation and detention methods). Either Spain or the US would prosecute as the jurisdiction in which the crime occurred, under the local rules. And in the US, it would actually be more likely a state crime, rather than a federal crime.
Crimes against US citizens can be prosecuted in the US, regardless of where they occurred, and will form the basis for the prosecution against the pirate the Navy just acquired. Spain may have a similar statute, but I don't know.
Lacking specifics of what you're trying to do, it's difficult to give you a solid answer. International law is exceedingly complicated.
Best of luck,
Jim Clark-Dawe