It was easy for him to climb through the window to commit the murder, but he couldn't remove the body that way.

After finding the key he unlocked the door, and put the key in the outside, intending to lock the door and remove the key as he left the room, so as to defer the discovery that Mr.Glenthorpe was missing until as long after his own departure in the morning as possible.

He may have found it a difficult matter to stoop and lock the door and withdraw the key while he was encumbered with the corpse, so left it in the door till he returned from the pit.

When he returned he was so exhausted with carrying the body several hundred yards, mostly uphill, that he forgot all about the key.

That is my theory to account for the key being in the outside of the door." "It's an ingenious one, at all events," commented Colwyn.