In the mid-2000s, German courts threw out a criminal complaint by Kaleck against then US Secretary of Defence Donald Rumsfeld over alleged torture in US military prisons.
Another obstacle is the "the difficulty of carrying out investigations on site", said Amnesty International's Sulzer. Instead, investigators are often forced to use satellite imagery, or get victims to draw the places where they were detained.
Cost and time are also factors.
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By AFP (Berlin) for France 24