The parents of 18-year-old Michael Brown spoke out Tuesday about the ongoing unrest in Ferguson, Missouri. Brown was killed by a police officer on Aug. 9, setting off protests and a swift police response.

On The Today Show, Brown's mother, Lesley McSpadden, said "justice will bring peace" and his father, Michael Brown, said he maintained faith in the system, though he added that it hasn't worked so far. Both parents called for the officer who shot their son, Darren Wilson, to face charges.

But the calls raise a tough question when it comes to police shootings: Who should investigate? It is often the police itself or a local law enforcement agency that leads the initial probe.

But that may be changing, at least in some places.

Wisconsin passed a law this year that calls for an outside investigator to step in when a police shooting ends in a death. That law was spurred by the 2004 police shooting death of a 21-year-old man in Kenosha, Wisconsin.

The man's father, Michael Bell, led that effort. He says it was a long, uphill battle, and one that was funded partly by a wrongful death lawsuit settled six years after the shooting.

Read Bell's article in Politico Magazine: What I Did After Police Killed My Son