Balko notes that "private police groups and the FBI keep close statistics on the number of cops killed and assaulted while on the job" but there is no national data on "the number of citizens killed by police":

The net result of all of this is a one-way flow of information that probably colors the way we think about the relationship between police and the communities they serve. When we get comprehensive data each year about the cops who were killed and assaulted in the line of duty over the last 12 months, but no data on how many people were killed and assaulted by police–justifiably or not–over the same period, it bends the debate toward more support for giving cops more power, more weapons, and more authority.