Adam Malka is assistant professor at the University of Oklahoma, and author of “The Men of Mobtown: Policing Baltimore in the Age of Slavery and Emancipation.”

As the nation mourns yet another group of shooting victims, many Americans have begun to take a closer look at the people doing the shooting. In Dayton, Ohio, the killer was a white man. In El Paso, the accused shooter is a white man. The person accused of fatally shooting 11 worshipers in a Pittsburgh synagogue in October is a white man. And the gunman who killed nine parishioners at a black church in South Carolina in 2015 is, like the others, a white man. Indeed, for years white men have been responsible for such massacres in far greater numbers than people in other demographic groups, and in some cases their rampages have been explicitly carried out in the name of white supremacy.

Why aren’t all of these mass shootings treated as acts of domestic terrorism? The United States is a country where terrorism has long triggered fear and invasive policies, yet law enforcement agencies have shown a persistent blind spot for white men’s violence. In a nation practically obsessed with law and order, white men’s violence continues to thrive without a sustained response.

The answer lies in the central role that white male vigilantes have played in the official policing process from the beginning, and particularly in the policing of racial minorities. Because violent white men have historically been the policing authorities, not the criminals, they helped to create a racialized legal system sympathetic to them and hostile to minorities — one that has proved deadly.

The linkage between police and white male violence dates to the origins of professional policing in America. Nowhere is this more visible than in Baltimore. Before the Civil War, Baltimore straddled North and South: It was the third-most-populous U.S. city and had the country’s biggest free black population, but it was also home to a few thousand slaves and a slave code. In this environment, white male violence was both protected by and essential to the growth of professional policing.

In cities such as Baltimore, there had been no official police before the 1820s. Instead, it was the white male citizenry that banded together to curb assaults, prevent theft, recover lost children, wrestle away hidden firearms, try killers, protect homes and businesses, apprehend arsonists, fight rowdies, quell riots and so on. During the 1830s and 1840s, Baltimore began introducing paid police officers, turning the job of surveilling criminals into a profession. But this did not prevent white male vigilantes from continuing to act as an arm of the state. One newspaper editorial put it clearly: A white man was “one of nature’s police officers, if not judge and executioner at the same time.”

This was especially the case when it came to policing black people. In antebellum Baltimore, ordinary citizens regularly intervened in interracial fights and detained black participants. They took it upon themselves to arrest black people they considered dangerous. They ran down alleged black thieves in the streets, arrested them and even occasionally served as prosecutors.

Municipal officials accepted this state of affairs as normal and legitimate, even as they began building professional police forces. In cases of arson, mayors went so far as to compensate the white men who arrested black suspects. White men policed Baltimore blacks often enough to blur the distinction between vigilantes and police officers. And vigilante practices shaped practices for these official police officers. An 1830s law instructed them, for instance, to “arrest and convey to the Watch house, all negroes or persons of colour” who violated the city’s curfew.

When it came to racialized mob violence, the professional police helped white mobs instead of stopping them. In the summer of 1838, white rowdies attacked the Sharp Street African Methodist Episcopal Church in west Baltimore, and police did nothing to protect the black victims. According to a newspaper report, they arrived only after “the mob had concluded their work and dispersed, so that no arrest took place.” The authorities never did find the assailants.

This wasn’t just ineffective law enforcement. Police enabled the violence to happen. The riot started after an altercation between a night watchman and a group of free black men. The white crowd had retaliated against what it considered an unprovoked attack on a police officer, acting as another arm of police enforcement.

Similar dynamics were at work when white men assaulted black caulkers on the Baltimore docks in the 1850s. Uniformed police officers didn’t just allow the violence as rowdies beat their victims with clubs, stabbed one man and chased others through the streets. Several of the officers on the scene were members of the gang that launched the attack.

Not every officer was implicated in this pattern of protecting white vigilantes and ignoring black rights. But a distinctive pattern was established: The city’s police force relied on white vigilantes as both official and unofficial law officers to regulate the activities of blacks, in frequently violent ways.

White men’s assaults on racial minorities would continue to earn official approval in the decades to come — not only in Baltimore but across the country, as well. When during the late 19th and 20th centuries African Americans heard the voice of their local sheriff underneath a Ku Klux Klan hood at lynchings, and when they smelled the smoke of their burning homes as officers watched in silence, and when they felt the blow upon their bodies of an officer’s baton, they would learn anew the twin lessons taught to Baltimore’s black caulkers: that the violence of white male vigilantes could be lawful, and that the law’s enforcers could act like a mob.

Law enforcement’s willingness to support white men’s racial violence has shaped our culture, leading, in part, to the presumed innocence of white men, even as official sanction for mob violence has receded over the past half-century. Scholars have shown that law enforcement has been much more inclined to presume the innocence of working-class whites and European immigrants than they have people of color; much slower to deal with white crime than black or brown crime; and much more likely to survey, patrol and target nonwhite communities.

These trends continue in today’s policing. They are most apparent in the overpolicing of communities of color — and in the blind spot our authorities have for angry white men. White men don’t face the same degree of surveillance and scrutiny as minority populations. This helps to explain why U.S. law enforcement failed to see the threat of white supremacy and why it doesn’t know how to stop the increasing number of violent white men who kill.

White men’s vigilantism has advanced a form of domestic terrorism over the past two centuries: it has been central to the state’s humiliation, condemnation and subjugation of minority communities. The links these perpetrators have had to the police establishment have thus muddied our definition of legitimate force, allowing white men to continue to inflict violence on society. We would do well to reckon with such history so that we may finally see the face of modern terror for what it is, and for what color it so often wears.