Having just reviewed a m/s from a talented young aspiring mystery writer, let me pass on the same (potentially deflating) advice: in all but two* (footnote below) "locked room" mysteries ever published, the writer has performed countless acrobatics to distract the reader from realizing that the Prime Suspect --and just as often, the perp-- is almost always the person who enters the room to find the body.
In most cases, the writer him/herself sincerely believes that he/she is the original creator of this "surprise" ending.
This is incorrect. It is believed the true First Creator of this "twist" was whoever did the cavern paintings found in Lascaux, France.
Use this knowledge to avoid the cliché. Or at least credit Oog, if you don't.
(* FOOTNOTE: And you know who you are, you enviably adroit mystery-scribes.)
-- Earl Merkel