Late on Wednesday night, nine people were murdered and one person was wounded while praying at the Emanuel African Methodist Episcopal Church in Charleston, South Carolina. Police have taken the suspected killer, 21-year-old Dylann Roof, into custody. The Justice Department has opened a hate-crime investigation into the shooting.

If these murders were racially motivated—and there’s ample reason to suspect they were—it’s significant that the shooter chose a church, and specifically this church, as a target of violence. “What African Americans see when they hear about this kind of violence being done in a black church ... [is] this whole pattern of violence dating back to the 18th century of attacks against their churches,” said Laurie Maffly-Kipp, a professor of religion and American history at the University of Washington in St. Louis. As my colleague Yoni Appelbaum wrote on Thursday, Emanuel African Methodist Episcopal Church is one of the oldest black churches in the South and has been an important part of the African American community in Charleston, particularly in the fight for civil rights. And as my colleague Conor Friedersdorf wrote, there’s a long history of violence against black churches in the U.S., particularly in the South.

To this day, churches are the center of social and political life in the American black community—and they are clear symbols of black influence and power. “To attack a black church in 2015 means that that young man, Dylann, understood that Emanuel A.M.E church was a focal point for not just the history of African Americans in the country ... but where they also mobilize,” said Anthea Butler, a professor of religious studies at the University of Pennsylvania.

There’s a cliched saying about religion and race in United States, something along the lines of “Sunday morning is the most segregated hour in America.” Black and white Christians still overwhelmingly tend to worship in separate congregations, for many reasons. “A lot of it has to do with the history of separation which led to different kinds of worship,” said Maffly-Kipp. In the early American republic, a number of black churches existed independently of white churches, “but they were pretty much shut down after the 1820s. Church Emanuel in Charleston was burned to the ground [in 1822] and shut down because of fears about the kinds of activities that were going on in churches.”

After it burned, Emanuel A.M.E wasn’t able to formally rebuild until 1865; it was closed by the South Carolina government. In the years leading up to the Civil War, independent black churches were outlawed, and African Methodist Episcopal congregations became concentrated in the North. But after the war, these churches were among the first to proselytize, Maffly-Kipp said. “There were missionaries in the A.M.E. church going down into the Southern states trying to get to black Methodists there.”

As blacks struggled to reconstitute their communities in the decades following the Civil War, churches became the center of African American life—largely, Maffly-Kipp said, because they didn’t have any other spaces they could fully claim as their own. Churches built schools, helped families raise their kids, and fostered political activity, particularly during the civil-rights era. “It’s the one safe space they have for decades that is out of the surveillance of white communities. It has a sacredness just by virtue of being a separate space,” she said.

In 2015, the de facto segregation in American churches is more complex than it was 200 years ago. “You see in white churches ... hourlong service on Sunday mornings, and in black churches, you have a three-hour services. … The style of the service is completely different.” But it’s also not just a matter of worship preferences. “Dylann was able to come in on a Wednesday night bible service and nobody asked any questions about him because black churches have always been open, even though they have also been targets of violence,” said Butler. “In white churches, especially in white evangelical churches, there is an expectation that you will become white. … You cannot bring your culture into that door.”

In a video from 2013, Clementa Pinckney, Emanuel’s pastor and one of the people murdered in the attack, said, “The African American church—and in particular, in South Carolina—really has seen it as its responsibility and its ministry and its calling to be fully integrated.” This may not seem like the right moment to consider that challenge—in such a stark moment of racial hatred, the obvious and most important response is solidarity and mourning. But the fact of racial segregation in American churches made Emanuel A.M.E vulnerable in a way that’s not paralleled in any other sphere of life—the black church is “the one symbol white people know about black people in this country,” Butler said. White Christians did not wish murder upon their black brethren, but it’s also true that the status quo of divided church life in America made it much easier for Roof to find a room full of black men and women to gun down during prayer. That is a deeper cultural context of these attacks, but it’s much more difficult to integrate America’s churches than to condemn horrific violence. “They are saying this is an attack on Christianity. Bullshit,” said Butler. “This is an attack on black Christians.”