General tips about locked room / impossible crime mysteries:
- Don't assume that everything that happens was caused by the killer.
- Don't assume that the seemingly impossible crime was the result of a perfectly executed plan. Very frequently it is the result of a plan that's gone wrong.
Read some John Dickson Carr / Carter Dickson. Especially _The Three Coffins_. (If nothing else, read chapter 17: "The Locked Room Lecture".)