Once again, a nation in mourning must try to make sense of mass murder. On Wednesday, a young white gunman walked into the Emanuel African Methodist Episcopal Church in Charleston, S.C., and — after sitting with an evening prayer group for nearly an hour — opened fire on those around him.

“I have to do it,” the gunman was quoted as saying. “You rape our women and you’re taking over our country. And you have to go.” When the shooting stopped, nine people were dead. On Thursday, Dylann Storm Roof, 21, was arrested in connection with the shootings, which the Justice Department said would be investigated as a hate crime.

Meanwhile, Americans have restarted the gun-control and mental-health discussions that typically follow these tragedies. And because the killer targeted blacks, they’ll keep agonizing over the racism that continues to plague the nation.

Among those killed Wednesday was the church’s pastor, the Rev. Clementa C. Pinckney, who was also a state senator and a well-known civil rights activist. Pinckney led a church with an important history. Mother Emanuel, as some called it, began in 1816, when several churches split from Charleston’s Methodist Episcopal Church. One of Emanuel’s founders, Denmark Vesey, tried to organize a slave revolt in 1822. But he was caught, and white landowners had the church burned in revenge. Emanuel is now believed to be the oldest black church in the South.

Wednesday’s attack came two months after the fatal shooting of an unarmed black man, Walter Scott, by a white police officer in neighboring North Charleston. That shooting was captured on video, and the officer has been charged with murder. As a result of that case, South Carolina lawmakers passed a bill intended to equip all South Carolina police officers to wear body cameras. Pinckney, 42, was a sponsor of that bill.

He was also a revered leader in his community, and was considered a rising star in the South Carolina Democratic Party. Known as a thoughtful human rights advocate and a moving speaker, in a 2013 speech he said that the struggle for justice is a fundamental element of his faith.

“That is what church is all about … freedom … to have equality in the sight of God. And sometimes you got to make noise to do that. Sometimes you may even have to die like Denmark Vesey … Sometimes you have to march, struggle and be unpopular to do that, ’’ Pinckney said.

On Thursday, a solemn President Obama told the nation that he had made statements about multiple slayings “too many times’’ during his presidency. He added that “hatred across races and faiths poses a particular threat to our ideals and democracy.” Once again, Obama said, “innocent people’’ were killed in part because someone “had no trouble getting a gun.’’ The president reiterated that “politics’’ in Washington have thwarted previous attempts to pass reasonable gun-control policies, but that Americans must “come to grips’’ with the problem and collectively shift attitudes about gun violence.

Democrat Hillary Clinton campaigned in Charleston before the shooting on Wednesday, and Republican Jeb Bush canceled scheduled appearances there on Thursday. When they and their competitors go back on the campaign trail, Americans should demand that they address gun violence and the racial divide.

How many more lives must be lost? How many more children in classrooms, shoppers in malls, politicians serving their constituents, worshipers in places of peace, and police and civilians on our streets must be gunned down before the nation — and its leaders — will do something meaningful in response?