Got to have the goods first

     "Does that mean he did it, Pat?"
     "It means he's an unreliable witness."
     "Do you know who did it?"
     "Who committed the two murders? I have a pretty good idea, but I haven't collected enough evidence to hook a fly. You don't just walk up to a murderer and say, 'You done it, you skunk!' unless you can prove it, Jean."

(from The Golden Box, by Frances Crane)