Recent statistics indicate that American police commit 1 in 5 homicides in which the victim is unknown to the assailant.
According to the DOJ, 14,200 Americans died as a result of homicide in 2013 (the most recent year with complete data). This represents a huge drop in the US murder rate, as it has fallen by half from 1994 to 2013, from 9 homicides per 100,000 Americans to 4.5 homicides per 100,000.
Of the 14,200 murders in 2013, we can estimate that roughly 4700 were random homicides, those committed by strangers unknown to the victim. FBI research has shown that almost 80% of murders involve some sort of acquaintance between the victim and assailant – we use a factor of 2/3 here to be more conservative . These 4700 random homicides are of particular interest because they invoke the greatest fear in the public – the fears of home invasion and carjacking murders, of murder-robberies on the street, and so on.
In reality these crimes have become rarer as the overall crime rate in the US has dropped. But while the crime rate has dropped, news over the past year of police homicides has exploded into the public eye. This raises a question – what percentage of random homicides are committed by police? FiveThirtyEight.com has covered this issue and reports that police likely commit at least 1240 homicides per year in the Unites States today. Since police interact with different members of the public every day, virtually every one of these homicides counts as a random homicide – the individual officer responsible for a given homicide doesn’t generally know the victim. This implies that US police officers commit roughly 20% of random homicides, even when justifiable homicides are taken into account !
If police are indeed committing 1 in 5 random homicides, that is deeply troubling. Perhaps it is justified by America’s overall murder rate? The Economist reports that German police killed 8 civilians last year (0 in the UK and Japan), versus 685 homicides in Germany. This leads to a ratio of 11.7 police homides per 1000 murders in Germany, versus 87 police homicides per 1000 murders in the US . American police kill civilians at 8 times the rate of their German counterparts, even after adjusting for country-specific murder rates. American police work is also safer as an occupation than it’s ever been – but the evidence shows that American police have failed to adapt to safer conditions, and appear to now be part of the homicide problem in the United States.
 The Bureau of Justice Statistics Homicide Trends in the US Report indicates (on page 16) that the victim had some acquaintance with the assailant in 78.1% of cases. This metric excludes the 44% of cases where data was unavailable (often because the crime remains unsolved). If we conservatively assume that the victim was known to the assailant in 50% of these cases, we get a blended rate of roughly 2/3 – therefore only 1/3 of all murders are random, in which the victim and assailant had no acquaintance.
 Taking the 1240 police homicides and dividing by 4700 random homicides yields a rate of 26%. Police departments reported 461 justifiable homicides in 2013, which would lower the police share of random homicides to 17%. But some percentage of police homicides claimed as justifiable may not be, and the 1240 police homicides per year may itself be an underestimate. If we assume that over 2/3 of all reported justifiable homicides were in fact justified, then this reduces the police share of random homicides to 20%.
 German police killed 8 civilians last year, divided by 685 homicides = 0.0117. Multiplying by 1000 makes this 11.7 police homicides / 1000 murders. In the US we have 1240 police homicides divided by 14200 murders, or 0.087, which equals 87 police homicides / 1000 murders. This metric shows that US police kill at 8 times the rate of German police, even after adjusted for the overall country-specific violence rate.