Dylann Roof, who is suspected of killing nine members of a Charleston black church on Wednesday, is officially under federal investigation for domestic terrorism. But commentators across the political spectrum aren't waiting for the official results of the investigation. They're already making a point of labeling the shooting a terrorist act. They have good reason to.

A White man enters a historic church of the Black freedom struggle, utters racist remarks, and murders 9 Black people. THIS IS TERRORISM. — Marc Lamont Hill (@marclamonthill) June 18, 2015

If Charleston and Oak Creek and countless other cities across America don't count as terrorism, what does? — Anil Dash (@anildash) June 18, 2015

By any meaningful definition the Charleston shooting was terrorism, full stop. — David Burge (@iowahawkblog) June 18, 2015

Making the choice to call this a terrorist act is a way of recognizing the long history of anti-black terrorism in America. For most of American history, the word "terrorism" has referred to acts committed by white people against black people.

In fact, anti-black terrorism perpetrated by the Ku Klux Klan was the reason for the first federal anti-terrorism law the US ever passed.

The first anti-terrorism law in U.S. history was the Klan Control Act, so really, this has been the definition of terrorism @jongardnersc — jelani cobb (@jelani9) June 18, 2015

A little background on that law (which is typically called the Ku Klux Klan Act or the Enforcement Act of 1871): in the late 1860s, racial terrorism in the South was ubiquitous. The Georgia Freedmen's Bureau recorded more than 300 murders of freedmen or assaults with intent to kill over 11 months in 1868. And the KKK, which had been formed by Confederate general Nathan Bedford Forrest, became an organized terrorist organization.

One of its goals was to intimidate and terrorize freedmen by attacking them, their homes, and their churches. The other was to intimidate or attack anyone who might vote for Republicans, the party of Reconstruction.

And this got especially bad in South Carolina after Republican victories in the election of 1870. From a report in the Journal of Negro History based on testimony about the Klan:

The Reverend A. W. Cummings, a Northerner who had been president of the Spartanburg Female College, compiled a list of 227 persons whom he claimed were abused by masked men in that country between the October election of 1870 and the following July 15. He asserted that some two hundred of this number had been beaten, seven wounded by gun fire and four killed. Squire P. Quinn Camp, a white office-holder, claimed that between September 2 and July 15 in the township of Limestone no less than 118 had been abused by the Klan in some fashion, of which four were shot, sixty-seven whipped and six had their ears cropped...So extensive was the fear engendered that whole sections of the rural Negro population slept in the woods for several months during the winter.

The federal government, led by President Ulysses S. Grant, decided it needed to step in to protect order in the South — and keep the political system from being overwhelmed by terrorist intimidation. So it passed a series of laws, including the Ku Klux Klan Act, which made it a federal offense to conspire to threaten elected officials and voters to deprive them of equal protection.

The Grant administration enforced the Klan Act aggressively, using federal militias and charging Klan members in federal court. The law is generally given credit for destroying the Klan in its first iteration as a national terrorist group (it resurfaced during the 20th century).

In the 21st century, terrorism is typically associated with Muslim extremism; when white people commit mass shootings, their ideology isn't as often brought to the fore. But because of the history of terrorism in the South, for many, labeling the Charleston church shooting terrorism is a way to recognize that black lives matter.