Three factors

"The opportunity for concealing the extent of our country’s collusion with those who have carried out the actual torture is increased by three factors: first, the nature of most of the techniques used (‘stealth methods’, so called); second, the choking powers of secrecy available to our government; and third, the haphazard way in which information about these matters emerges, when it emerges at all, which hampers our ability to ask the most basic questions." (thanks Laleh)