If one example doesn't do it, and I can't blame you for that given the conceptions people have about our media and such, here's a well researched report on it.
Human Rights Watch
Shielded from Justice: Police Brutality and Accountability in the United States
http://www.hrw.org/reports98/police/An excerpt from it
The brutality cases examined, which are set out in detail in chapters on each city, are similar to cases that continue to emerge in headlines and in survivors' complaints. It is important to note, however, that because it is difficult to obtain case information except where there is public scandal and/or prosecution, this report relies heavily on cases that have reached public attention; disciplinary action and criminal prosecution are even less common than the cases set out below would suggest.
Our investigation found that police brutality is persistent in all of these cities; that systems to deal with abuse have had similar failings in all the cities; and that, in each city examined, complainants face enormous barriers in seeking administrative punishment or criminal prosecution of officers who have committed human rights violations. Despite claims to the contrary from city officials where abuses have become scandals in the media, efforts to make meaningful reforms have fallen short.
The rarity isn't the one that escapes attention, the rarity is the one that gets attention. That from personal experience followed by years of research into the system. If a tape isn't running and witnesses don't come forward it didn't happen, and the cops know where the tapes and cameras are most of the time.
Don't get me wrong, many are honorable people who just want to do the right thing. But, the system doesn't stop the bad ones, it protects them. If you've the time I've got a video link showing some stuff that happened in our prisons as well, looks an awful lot like Abu Gharib at times.
http://informationclearinghouse.info/article8451.htmThe rarity isn't it happening, it's us hearing about it.