This paper presents a novel mediated reality system designed to support collaboration between crime scene investigators during a first analysis on a crime scene, remotely supported by expert colleagues. Requirements elicited from interviews and interactive sessions with practicing crime scene investigators, provided the design criteria for a head mounted display capable of real-time map-making for spatial collaboration. The resulting system is evaluated in a staged crime scene experiment in which lay investigators collaboratively solve a spatial problem with remote experts. The results show that our novel approach to remote spatial interaction with the physical scene enables investigators to tackle current issues on site in collaboration with experts at a distance.