There was little violence after the acquittal of Trayvon Martin's killer last July. Peace prevailed when at least four other unarmed black males were killed by police in recent months, from New York to Los Angeles.
Then 18-year-old Michael Brown was gunned down in Ferguson, Missouri. Waves of rioting have convulsed the St Louis suburb for more than 10 days.
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 organised peaceful protests after Florida neighbourhood watch volunteer George Zimmerman was acquitted in Martin's killing.
Powell is headed to Ferguson as an organiser and peace activist after the killing of Brown, 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 armoured military vehicles, assault weapons and dogs, it reminded Powell of the 1960s civil rights movement.
"Zimmerman was one person. This is an entire police force," he said. "It feels like the whole system doesn't care."
There are no reliable national statistics on people of any race killed by police. One study, relying on internet searches of media reports, found 18 unarmed black people killed by police and security personnel in the first three months of 2012.
Some of the rioters, according to media reports, are hardened, violent young men who speak of seeking "justice," which is often confused with revenge. Some are coming to Ferguson from out of town, whether to show solidarity or fight the crackdown, or drawn to the media spectacle. Police have reported arrests of people from New York and California.
"It feels like a turning point," said Blair Kelley, a history professor at North Carolina State University. "Because so many black men die at the hands of the state."
Kelley and Powell both said that the nature of Brown's killing fuelled anger: he was shot at least six times in broad daylight, in the middle of the street, in his own housing complex. Then his body lay in the street for hours, uncovered, in a pool of blood.
"There were more than 100 people there looking at his body," Kelley said.
She mentioned the killings of Jonathan Ferrell, an unarmed black man who was shot by a white officer after crashing his car in North Carolina last September, and a black woman, Renisha McBride, who crashed her car in Detroit, went to a nearby house, and was shot dead through the front door.
"Those happened at nighttime, away from the public gaze," Kelley said. "To leave Brown in the street like that, it was a disregard they could feel … and see."
The last US 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 California. Hundreds of businesses were damaged, cars were overturned and smashed, and more than 100 people were arrested.