The average homicide clearance rate — cases solved by police departments compared with the number of known homicides — which approached 90 percent in 1960 is now a third less, 61 percent.

First and foremost, the reason why cold cases are so difficult to solve is that witnesses and suspects die or become incapacitated due to age and illness. Memories of the events begin to fade. Some witnesses or suspects move away.

And according to some experts, solving cold case homicides relies more on the emergence of new witnesses than on the DNA analyses and other forensic techniques celebrated in crime dramas.

In one study, the factors linked to successful convictions in 189 cold case investigations from the files of the District of Columbia’s Metropolitan Police Department were looked at. The homicides dated back to the 1970s, but most had occurred in the past two decades.

Of those reopened cold cases, 24 percent ended in convictions and 24 percent were cleared by “exceptional” means, instances in which the culprit was already dead, in prison, or had gone missing. The rest remained unsolved.

What made for a cold case conviction? New witnesses helped resolve 63 percent of the cleared cases, the survey found. Often an ex-girlfriend or ex-friend of a murderer came forward years after a crime. DNA matches figured in only 3 percent of the cleared cases. “The worst reason to reopen a case was because of family pressure, if you want a conviction,” one cold case detective said.

Although this post was on the difficulties involved in solving cold cases, I can’t help but finish on a positive note. On the bright side, cold cases — if there is a dedicated unit — have a couple advantages. According to Kenneth Mains, President of the American Investigative Society of Cold Cases (AISOCC):

The original investigator has the best chance of solving the case because he or she has the best chance to interview people when things are fresh in their minds. The original detective also has the benefit of seeing fresh abrasions, bruises, and scratches on people that a cold case investigator doesn’t see. However, there are two major advantages that a cold case investigator has over the original detective. The cold case detective is not buried by case load and cold cases can be worked at a leisurely pace with no interruptions.

Detectives do not have the luxury of only working a homicide case. When a homicide occurs and is assigned to a detective, his other work doesn’t stop. The new homicide takes priority, but his case load continues to pile up on his desk. The robberies, the burglaries, the frauds continue to come in. The detective works those homicide leads until she reaches a dead end and her supervisors remind her about the other cases that are piling up. That is how a case becomes cold. It gets pushed to the side after leads dry up.

A cold case detective doesn’t have that problem.

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