Last week, in the course of his 53rd traffic stop in 14 years, Philando Castile was killed, for reasons that remain largely unexplained. Gov. Mark Dayton said the following day that if Castile had been white, he likely would still be alive. Instead, his death has occasioned another outpouring of pain over racial injustice.

The events of this summer echo back to 1967, when 75 American cities experienced major racial disturbances — often triggered by police brutality. A presidential commission created to study the crisis laid the blame squarely on a system of racial apartheid designed to separate black families from white society. In its words:

"Our nation is moving toward two societies, one black, and one white — separate and unequal. Segregation and poverty have created in the racial ghetto a destructive environment totally unknown to most white Americans. What white Americans have never fully understood … is that white society is deeply implicated in the ghetto. White institutions created it, white institutions maintain it and white society condones it."

The commission understood that the suffering of black Americans was intrinsically bound up in segregation. And it understood that segregation was not merely a system of separation, but a system of dominance, through which black Americans were held at a remove by powerful economic, social and political forces, including law enforcement.

The commission urged the country to reverse course. At first, Minnesota was at the forefront of this work. Between 1968 and 1985, the Twin Cities adopted proactive school and neighborhood integration policies that reduced segregation and closed racial disparities, building the groundwork for prosperity, stability and equal treatment for all.

But as the sense of crisis diminished, integration efforts slowed. Plans to maintain diverse schools and to ensure that housing was mixed by income were eliminated. A seductive myth took hold: the idea that, like distant colonies, segregated neighborhoods could be lifted out of poverty and despair by a concentrated program of uplift, without troubling the region's white enclaves, which were already well-fortified against poor or nonwhite intrusion.

Segregated neighborhoods once described as racial ghettos, deliberately invoking Europe's bitter history of confining unwanted social groups, were euphemistically retermed "RCAPs" — racially concentrated areas of poverty. The existence of such places is now often taken as inevitable.

Sociologist Kenneth Clark once described inner-city neighborhoods as "colonies of white society" where "social agencies are financially precarious and dependent upon sources of support outside the community." The Twin Cities area often has seemed determined to prove him right.

In fact, there has been a precipitous increase in segregation in the Twin Cities. The number of very racially isolated schools has risen more than sixteenfold in two decades. Segregated neighborhoods have grown rapidly in size and population. The region now produces the nation's largest disparities between blacks and whites, with immense gaps in employment, education, wealth and health.

Meanwhile, many suburbs have closed their doors to affordable housing. Exclusionary zoning has been employed to restrict the supply of homes in affluent areas, and school boundaries are carefully adjusted to ensure that the most privileged schools are insulated from problems elsewhere in the region.

With every passing year, white and black Minnesotans live in more divergent worlds.

Stuck bridging this divide are the police. Segregation means that most officers in poor neighborhoods have little personal connection to the area they are policing. To residents, this can make officers feel like occupiers. In white areas, segregation can cultivate the idea that the duty of the police is to enforce borders.

In comments last week, President Obama acknowledged these realities and echoed the decades-old presidential commission: "We are asking police to man the barricades in communities that have been forgotten by all of us for way too long."

Minnesota has grown more adept at keeping nonwhite citizens geographically confined. But the consequences of maintaining a racial caste system cannot be as easily contained. The irony of apartheid is that the act of retreating into privileged enclaves creates inequality and injustice from which nobody can escape.

When he was shot dead by police, Philando Castile was not in a poor or segregated neighborhood. He was passing through the middle-income, predominantly white city of Falcon Heights, and he was pulled over by a police officer from the middle-income, predominantly white St. Anthony. In the end, it didn't matter.

It didn't matter either that Castile himself was a well-loved employee at an integrated school, someone whose life had not been lived within the prescribed bounds of racial segregation.

In a segregated society, everybody is in danger of being reduced to their race and their race alone. Although the reasons for the shooting are still contested, a black man crossing through a white area is at risk of being seen as an interloper, a suspect. Even the merest imagined hint of a threat — the word "gun," said aloud — can be enough to incur fatal force.

In the final moment, all that may have mattered was that Castile was not where the gatekeepers of segregation thought he belonged.

Even as our divisions grow, many wonder if racial enclaves are the inevitable byproduct of a natural, neutral process. But Castile was killed crossing the border between two worlds last week. And natural borders do not need armed guards.

Myron Orfield is a professor of law at the University of Minnesota Law School and director of the school's Institute on Metropolitan Opportunity. Will Stancil is a research fellow at the institute.