Like the morning New York City cops murdered high school senior Timothy Stansbury, Jr. On January 24, 2004, two cops were patrolling the rooftops of a Brooklyn public housing complex. Stansbury and his buddy were on their way up the steps to the same rooftop when Officer Jason Hallik opened the door so that his partner, Officer Richard Neri, could point the barrel of his service revolver down the stairwell and check for trouble. Neri saw the unarmed Stansbury and opened fire, killing him. Neri, an 11-year veteran, later testified that Stansbury had scared him; a grand jury said that fear made the murder an accident and declined to indict.

And of course, we all know about Amadou Diallo, perhaps the most infamous example of New York City cops letting their shaken nerves get the better of them. The 23-year-old Diallo was shot 41 times on his Bronx doorstep by four plain-clothes officers. He was unarmed, but the elite street-crime unit fantasized that he was grabbing a gun when he reached for his wallet. Once again, the cops' bizarre perceptions about Diallo's threat were enough to clear them of wrongdoing.

Jesse Jackson says these killings form a pattern that demands the Justice Department step in and retry Detectives Marc Cooper, Gescard Isnora, and Michael Oliver in Bell's killing. He's probably right. But the pattern stretches wider, and farther back, than the NYPD. American law has been sanctioning the killing of black people to mollify white fear for centuries.

Throughout the colonial era, whites lived in perpetual terror of the Africans they'd imported. Even the bonds of chattel slavery weren't enough to soothe their unease, so waves of vigilante justice were needed as well. Any unrest spawned massively outsized responses of state-approved violence. When law enforcement in colonial South Carolina uncovered a 1740 plot by a few dozen slaves to steal weapons and flee, they hung people at the rate of 50 a day. A year later, a burglary investigation in New York City so spooked law enforcement, that they ended up burning alive or hanging 32 slaves, and deporting another 72.

Then and now, white paranoia about black violence is the emotional toll America pays for the racial caste system it has built. It still plays out in all parts of our society, from the grotesque gangster caricatures white record labels promote to the coded language yuppies use to identify neighborhoods that are "safe" for them to venture into. But nowhere does it show up more clearly than in police operations in starkly segregated ghettos like those in which Bell, Stansbury, and Diallo were gunned down.

As a ColorLines magazine investigation documented last fall, blacks accounted for 66 percent of those killed by New York City police between 2000 and 2007 (New York is a perennial leader in police fatalities, averaging 12 a year over those years). And while the violent crime rate plunged to historically low levels in that time period, the number of people killed by police has not budged—indeed, the number of cop bullets fired has skyrocketed. And it's happened with impunity. Out of 88 fatal shootings, including at least 12 in which victims were unarmed, in only one instance was an officer convicted of criminal wrongdoing.

Much has been made of the fact that two of the three detectives who shot Bell and his two pals were people of color. It's significant that the one white cop, Oliver, fired 31 of the 50 shots, and that the black cop, Cooper, is the only one to have apologized to the Bell family. But the reflexive assumptions of threat that drive the death-by-cop racial disparity are systemic rather than individual. Black and Latino cops like Cooper and Isnora operate within a bureaucracy that not only condones but encourages them to see black men as combatants. Take, for instance, the public housing patrols that got young Stansbury killed. Police so often have their guns drawn while conducting rooftop-to-stairwell searches that, according to ColorLines, the fire department sent a safety memo to its members warning them to "loudly announce" themselves when coming up stairwells so cops won't shoot them. It's hard to imagine a similar scenario in a white neighborhood, even one with the same socio-economic makeup..

This culture of irrational fear is just the sort of thing that Barack Obama tried to point out when he talked about his grandmother's fear of black men. And the appalled response of some white folks in the media and the general public is telling. Rather than consider the nuance and implications of Obama's example, many obsessed over whether he could fairly call her thoughts "typical" of whites. Of all the no-go zones in America's supposed racial discourse, the white fantasy about the threat black men present is perhaps the most inviolable. As long as that remains the case, cops will continue to get free passes for shooting us down in frightened, reckless fits. They need only to cover the murder over with some threadbare lie of perceived threat.

Kai Wright is author of the newly released, "Drifting Toward Love: Black, Brown, Gay and Coming of Age on the Streets of New York."