US law has a concept of accessory-before-the-fact. Assisting in the concealment of a crime is in itself a crime. Unlike an accomplice, an accessory need not be aware of the specifics of the crime. This was a big stick wielded by US law enforcement against pre-paid phone operators in the early 2000s to compel cooperation.

But you are really missing the forest for the trees here. If the crime against the US is serious enough, extra-judicial rendition is on the table. So is extra-judicial execution. https://en.wikipedia.org/wiki/Assassination_of_Qasem_Soleima...

The idea of being based in a specific country giving you some sort of immunity is disingenuous.