These are good riddles for yes-no responses. As is, vague, not clearly stated with enough specifics. I'm assuming there is only one way to get into and out of the room . . . the door, correct? Considering the Police had to force it open and so was locked from the inside, don't you mean "how was the murderer able to bolt the door from the outside after leaving?" Otherwise, there is no reason why he couldn't have just walked out.
What kind of bolt . . . metal latch? Knob turn? Drop bar?
Is the ash on the inside or outside of the doorway?