The profile hints at someone that is a traveler. Could the killer have been someone in construction? We don't know how old the complex was at the time or if it had undergone extensive renovations in the past. Someone knew or found out about the common attic space. Someone in construction? City permits? Who built the complex? The killer may have not worked on this complex, but they worked for the construction firm that built it and that firm used that design elsewhere so the killer was familiar with it.