This Sociological Life

"Circumstantial," said Pryor's solicitor, materialising with suspicious speed. "A consultancy declared through proper channels. An access log that proves presence, not poisoning. A memo about methodology, which academics write about each other constantly, usually without anyone dying afterwards." He turned to Rook. "Is this really sufficient, Sergeant?"

You could see Rook's certainty thinning, grain by grain, in exactly the way you imagined it must thin for considerably more experienced detectives than him.