Here's some more interesting information from a study done in 1999 about clearance of homicide cases:
In 68% of all closed cases, the victim knew the offender.
In 30.2% of all closed cases, the offender was a stranger
In only 11% of open cases, victim knew the offender
Cases most easily solved are those where the killer is related to or has a relationship with the victim.
Only 3.9% of murder cases are solved after more than 1 yr.
Offender variables that affect whether a murder is solved - those offenders least likely to be caught are those who are drug buyers and who have a previous record of property crimes
Table 17 shows the circumstance most likely for a murder to go unsolved - when it's punishment to the victim for informing on crimes
Lots of good information at the link.
Another good article on the topic from The Economist, last year
http://www.economist.com/news/unite...rest-more-third-nations-killings-getting-away
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ETA: Here's an NPR article with good information. It contains a link where you can search different localities and find the statistics for how well they solve murder and other cases.
http://www.npr.org/2015/03/30/395069137/open-cases-why-one-third-of-murders-in-america-go-unresolved