This is from the US Department of Justice website – and the process of having complaints heard and acted upon is much more complex than is listed here but this may be useful as a primer.

Complaints about Law Enforcement Agencies
What you should know before filing a complaint about a Law Enforcement Agency:
The Section has authority to investigate only state and local law enforcement agencies. This means that the Section cannot investigate federal law enforcement agencies (such as the Federal Bureau of Investigations or Drug Enforcement Administration). Complaints regarding federal law enforcement agencies may be directed elsewhere.
The Special Litigation Section does not bring criminal cases against those who violate the civil rights of citizens. Criminal complaints can be made to the Criminal Section of the Civil Rights Division.
The Section is authorized to take action only if there is a pattern or practice of illegal conduct. While we do collect information about specific incidents in order to determine whether there may be a pattern or practice that warrants investigation, we do not have authority to investigate or remedy individual complaints about a particular incident.
Even if your complaint falls outside the authority of the Special Litigation Section, there may be other agencies that could help you. A partial list of other federal agencies that respond to similar complaints is listed below.
How to File a Complaint:
The most effective means of filing a complaint is to write a letter to the Section explaining the situation about which you are complaining, with as much detail as possible. If you are aware of similar incidents involving others, please include that information as well. Please include information on how to contact you if we need further information (such as an address and telephone number). Also, do not include original documents as we cannot guarantee their safe return. Address all complaints to:
Special Litigation Section
U.S. Department of Justice
Civil Rights Division
950 Pennsylvania Avenue, NW
Special Litigation Section
Washington, D.C. 20530

The Section can also be reached by telephone (202-514-6255 or toll-free at 877-218-5228) or fax (202-514-0212 or 202-514-6273).

Answers to other frequently asked questions about the complaint process can be found on our Frequently Asked Questions page.