this is the 254th in my series of forgotten or seldom read books
The Department of Dead Ends by Roy Vickers, © 1949, my copy: a 1955 Penguin paperback
An investigation is made but the files find their way to the Department of Dead Ends, the repository of cases not solved, clues which led nowhere. Then eventually, sometimes many years later, some small fact or occurrence comes to the attention of Inspector Rason who finds a connection and solves the case.
I got this paperback second hand some years back and let it sit on the shelf for some time before picking it up to read. That was foolish. Once started I couldn’t put it down. These stories are terrific!