The Guardian has been on the forefront of investigative, as opposed to reactive, journalism about police killings in America.  For instance, they were the ones who created and maintained a nation-wide database of police killings, something that didn't exist before.

Today there's a superb investigative story on Kern County, California, which has the highest per-capita rate of police killings in the nation.   As is commonplace, investigations into police misbehavior are handled by the department itself, and are often perfunctory at best.