I love true crime documentaries and for me this is the bench mark. This is why we pay the tv license because the BBC is capable of quality programmes like this. No annoying voice over explaining things like you're 5 years old.
This documentary is quality not just on the subject matter but the cinematography, the music/soundtrack which create the perfect dark atmosphere for something so dark really as it doesn't get much worse then the state killing people someone of whom are likely innocent.
I think the makers also made another top series called 'Love and hate' which has one of the scariest documentary episodes I've ever watched.