Officially called “separation,” the procedure physically isolates a prisoner so that he cannot communicate with anyone but U.S. personnel, such as guards, interrogators, and medics. Human rights advocates, including the United Nations special rapporteur on torture, have denounced separation as solitary confinement by another name — a charge the U.S. denies — and say it could amount to inhumane and illegal treatment all by itself.

But often, the documents show, separation was approved for use in combination with other interrogation techniques, raising additional alarms.

Under the government’s interrogation rules — tucked away in an addendum to the Army Field Manual called “Appendix M” — separation can be imposed together with tactics such as restrictions on sleep and psychological approaches including a technique called “Fear Up,” which heightens fear to get prisoners to talk. Human rights advocates say that the combination of methods could easily add up to abuse or even outright torture, which the U.N. defines as inflicting “severe pain or suffering, whether physical or mental,” to make a person talk.

In response to detailed questions, a Defense Department spokesperson wrote that interrogators apply separation in compliance “with all applicable standards of humane treatment” and that they must obtain special permission to use it “on a case-by-case basis when there is belief/concern the detainee possess [sic] information of potential intelligence value.”