There is no federal database that tracks violent encounters with the police, much less incidents in which civilians die at the hands of law enforcement. So a handful of citizen activists are trying to keep track of the data themselves.

http://america.aljazeera.com/watch/shows/Ali-Velshi-On-Target/articles/2015/5/28/No-data-on-how-many-killed-by-police-in-America.html