> I am anxious however to learn more on the subject if you can provide additional evidence that legally the role of the US Federal government is to act as an oversight body for the states.

The Federal government has the power to apply consent decrees to specific police departments when those departments are found to have used force excessively, per the 1994 Violent Crime and Law Enforcement Act[0]. This essentially opens police departments up to lawsuits if they exhibit a pattern of bad behavior. In fact, this law explicitly requires DOJ to issue an annual report on "the use of excessive force by law enforcement officers", although they haven’t.

0: https://en.m.wikipedia.org/wiki/Violent_Crime_Control_and_La...