From The Observer:

"After the recent publication of photographs of Iraqi prisoners being tortured at Abu Ghraib, and the admission by the US administration that a range of coercive methods were authorised for interrogators in the war on terror, a German court would need firm evidence that the statements were truly voluntary, the official went on."

So instead of producing evidence about the crime, evidence about the way evidence has been collected is now needed (meta-evidence if you will). If this were a flesh-and-blood crime (i.e. with a murder weapon, physical clues and so on) as opposed to merely conspiracy to murder, there would have been a chain of evidence to maintain to prove the prosecution's case.