gsrLOVE, you're close. It's not per se about past cases, the cases dealt with in the episodes were not solved (Organ Grinder: they didn't know if the wife or the secretary did it; The execution of Catherine Willows: they didn't know in that episode who the "blue-paint killer" was and in Built to Kill pt 2 & Post Mortem the Miniature Crime Scene Killer)
Ya the Execution of CW is the tough one in that group because the case was solved, and Blue Paint Killer decided to kill again which is a little different from the other three.