The Assembly passed the bill in February. The Senate approved it on a voice vote Tuesday, the last day of the two-year legislative session. The measure now heads to Gov. Scott Walker.
The bill comes in the wake of several high-profile officer involved deaths across the state in recent years. Supporters say it will alleviate concerns about investigators from the same department covering up their friends' misconduct.
Smaller departments routinely rely on outside agencies to investigate officer-involved deaths. But the state's two biggest departments, Madison and Milwaukee, investigate their own officers.