United States law prohibits torture. I would imagine it would not be a defence for a US citizen involved in torture to claim it was done outside US territory, if it took place in a US government (military or CIA) facility.

Surely a test prosecution could be undertaken against someone low down the foodchain, with a view to encouraging co-operation in prosecuting more significant offenders?

Is it possible to bring a private criminal prosecution before a US federal court, or will this strategy have to wait for a Democratic administration?