Yeah this is especially true with Detroit. It has to do with the system as well. Police are not very involved in the community they're policing nowadays, often living in cities outside of where they work. Murder cases rely heavily on witnesses and testimony (like a snitch for example) if there isn't much physical evidence or the physical evidence is inconclusive.

Because police aren't very involved in their communities, witness testimony and snitches are hard to come by. Especially in a place like Detroit. Murders go unsolved here all of the time and the last time I checked (when I was living there 6 years ago) the MAJORITY of murders went unsolved.

Edit: Being involved in your community as a police officer can also help you identify things like motives or tensions between groups or gangs.