The response to Brown's death turned violent because of a convergence of factors, observers say, including the stark nature of the killing in broad daylight, an aggressive police response to protests, a mainly black city being run by white officials — and the cumulative effect of killing after killing after killing of unarmed black males. "People are tired of it," said Kevin Powell, president of the BK Nation advocacy group, who organized peaceful protests after the Florida neighborhood watchman George Zimmerman was found innocent in Martin's killing. Powell is headed to Ferguson as an organizer and peace activist after the killing of Brown, the unarmed black 18-year-old who was shot by white police officer Darren Wilson. Battles have raged in Ferguson almost nightly, with stores looted, police firing tear gas and rubber bullets, people tossing Molotov cocktails, and dozens of arrests. When police first confronted protesters with armored military vehicles, assault weapons and dogs, it reminded Powell of images from the 1960s civil rights movement. There are no reliable national statistics on people of any race killed by police, but anecdotal reports count significant numbers. Some of the rioters, according to media reports, are hardened, violent young men who speak of seeking "justice," which is often confused with revenge. The last riots over an unarmed black death were in 2009, when Oscar Grant was killed by a white officer while lying face down on a train platform in Oakland, California. When people really feel vulnerable in the face of police violence and local authorities are totally impervious to community demands.