Pastor Julius McDowell embraces Reverend Brenda Kneece during a prayer vigil Thursday in Charleston, S.C.’s, Morris Brown AME Church for victims of Wednesday's mass shooting. (Curtis Compton /Associated Press)

“UNDESCRIBABLE.” “UNFATHOMABLE.” “Unspeakable.” “Beyond incomprehensible.” Charleston officials struggled Thursday morning to find words to describe the shooting at a historic African American church in which nine people were killed. They were not alone. Much of America was grappling with the calculated murder of people gathered for prayer in a house of worship. That the gunman was apparently motivated by racial hatred sharpened the horror.

The first response to Wednesday night’s massacre at Emanuel African Methodist Episcopal Church must be to think of the victims, their families and their community. Six women and three men — including the church’s pastor, the Rev. Clementa Pinckney, who was also a state senator — were killed and at least one person was injured by a white male assailant who sat through an hour of the prayer service and then opened fire, allegedly making racial comments. “The heart and soul of South Carolina was broken,” South Carolina Gov. Nikki Haley (R) said of the grieving of families and pain of the community.

It was also natural for there to be relief that the suspect, identified as 21-year-old Dylann Roof, was quickly apprehended. His arrest followed an extraordinary mobilization of law enforcement and a tip from a good citizen, both reflecting the outrage over this heinous act.

But even as the nation reels and mourns, there must be consideration of how such a crime could occur. That a person so young could feel — and act out — such hatred is a sad reminder that the United States, for all its progress, is far from closing the book on the racial divisions that warped much of its history. Equally perverse is how easy this country makes it for guns to fall into the wrong hands. Good that both Charleston Mayor Joseph P. Riley Jr. (D) and President Obama gave voice to that issue Thursday.

Mr. Riley was visibly angry in talking about the lack of a major national effort to tighten gun laws. The president, by contrast, appeared weary, and little wonder considering how often he has had to speak to the nation after mass shootings: 13 people killed at a Binghamton, N.Y., community center; 13 people killed and another 30 shot at Fort Hood in Texas; six people killed in Tucson in a shooting that grievously injured a congresswoman; 12 people killed and 58 wounded in a Colorado movie theater; 20 Sandy Hook Elementary School students and six of their educators killed. Nor is that even a complete list.

“At some point,” Mr. Obama said, “we as a country will have to reckon with the fact that this type of mass violence does not happen . . . in other places with this kind of frequency.” Expanding background checks or shutting down unscrupulous gun dealers would not prevent every incident, but they are sensible steps that would reduce the violence. But, as Mr. Obama said,“the politics of this town foreclose a lot of those avenues right now.” So instead of acting, we add one more name to a roster of grief and outrage: Tucson. Aurora. Newtown. Charleston.