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Protests continue following the Aug. 9 shooting of unarmed teenager Michael Brown by Ferguson police officer Darren Wilson. The marchers, though, are not just protesting Brown's slaying. They are also voicing pent-up anger at an old problem: police violence, often directed at black and brown people.

The horrific beating of Rodney King by five police officers in Los Angeles in 1991 -- and the subsequent acquittal of his assailants -- sparked the L.A. riots of 1992, leading to 53 deaths, some at the hands of police. It was also a video introduction to police brutality for those in America who may have doubted its severity.

Twenty years later, a police beating or shooting has a decent chance of getting caught on camera -- either the one on the phone in everybody's hand or the surveillance camera pointing down at the street. The latter captured Kelly Thomas, a schizophrenic homeless man, being beaten to death by authorities in Fullerton, California, after being mistaken for a suspect in a series of car break-ins in the area. They, too, were acquitted.

Footage shows Oscar Grant being restrained by BART transit officers on the train platform in Oakland, California, following an altercation. Unarmed and lying on the platform, Grant was shot to death by James Mehserle, who claimed to have mistaken his gun for his taser. The alleged accidental death of Grant at the Fruitvale BART station was memorialized in last year’s film Fruitvale Station.

In June, Edgar Vargas Arzate was running from police in Santa Ana, California, near where Thomas was beaten, before surrendering in the front yard of a neighbor’s home. He was lying unarmed and face-down in the grass, but officers still savagely beat Arzate. When he was taken into custody, he was charged with assaulting an officer.

In July, Staten Island resident Eric Garner was suspected by the NYPD of selling untaxed cigarettes. When he refused arrest, an officer put the asthmatic man in a chokehold. Garner repeatedly screamed "I can't breathe!" and died soon after.

In August, less than two weeks after the death of Michael Brown, police in the St. Louis area shot another man. Officers responded to a 911 call regarding an alleged robbery at a convenience store. When they arrived, footage shows Kajieme Powell pacing, and he yells "shoot me now." Officers said he had a knife. Within 15 seconds of arriving on the scene, the two officers opened fire, killing Powell.

These are only a few examples of the force employed by police officers across the country. Some experts contend that police are trained to shoot to kill. A recent Washington Post op-ed written by an officer told readers, “[I]f you don’t want to get shot, tased, pepper-sprayed, struck with a baton or thrown to the ground, just do what I tell you ... Most field stops are complete in minutes. How difficult is it to cooperate for that long?”

Others are calling for a new law that would require law enforcement to wear cameras to avoid police misconduct and maintain a higher level of accountability. Studies in which officers have been asked to wear cameras have shown the method can be effective -- one California study found police brutality plummeted when cops were recorded.

The fact remains that we do not know how many people are killed in police shootings annually, but we do know that at least five unarmed black men have been killed by police in the last month alone.

As Andrew Sullivan points out, it doesn't have to be this way: