This article originally appeared on The Trace.

A shooter fired into a crowd of people last night at a country music festival outside the Mandalay Bay casino on the busy in Las Vegas strip, killing “in excess” of 50 people. More than 400 were injured.

Authorities say the gunman acted alone, firing on the crowd from a hotel room above. He died from a self-inflicted gunshot wound as a SWAT team stormed his room, authorities said.

The incident is the worst mass shooting in U.S. history.

There is no official definition of “mass shooting.” But it is often understood as an incident in a public place that claims four or more lives, and attracts widespread media coverage. In the last five decades, these events have become far more common.

Other groups use a much broader definition for what counts as a mass shooting, sweeping in incidents that happen in homes, and where there are four or more casualties — not just deaths. Gun Violence Archive tallied 385 mass shootings using this broader definition in 2016, resulting in at least 457 deaths and 1,546 injuries.

The random nature of indiscriminate gunfire unleashed without warning is all the more frightening because it can happen anywhere. Just in the past four years, gunmen have massacred worshipers at a church, moviegoers at a theater, people at a gay nightclub, and young children at an elementary school. In July 2016, a 25-year-old Army reservist who was reportedly angry over police shootings of unarmed black men killed five police officers and wounded 11 others during a rampage in Dallas.

Mass shootings are both tragedy and spectacle. As a result, they attract a huge amount of attention, which tends to distort views about the prevalence of incidents, the most common victims, and how the weapons that are used are obtained.

Here is our guide to understanding mass shootings in America.

By one definition, mass shootings are a daily occurrence in the United States.

The FBI does not count “mass shootings,” but defines “mass murder” as an event in which four or more people are killed — excluding the perpetrator, and not including domestic violence incidents — at one time.

All but three of the ten deadliest mass shootings in modern American history have occurred within the last decade.

Despite the attention they garner, mass shootings account for just 2 percent of gun deaths.

Roughly two out of three Americans who die from a gunshot wound commit suicide, according to the latest federal data.

The majority of mass shooters obtained their weapons legally.

Omar Mateen, the gunman who killed 49 people at the Pulse nightclub in Orlando, purchased the rifle and handgun he used in the assault from a federally licensed firearms dealer. Syed Farook purchased two of the handguns used in the San Bernardino massacre from a California gun shop.

An analysis of recent large-scale mass shootings by The New York Times found that 13 of 16 purchased their guns in a similar fashion — legally, after undergoing a background check administered by a federally licensed dealer.

Here’s a step-by-step guide to what that entails.

Would a ban on assault weapons curb mass shootings?

Probably not. Many experts believe it is not what the gun looks like that matters most — it’s how many rounds it can fire without reloading.

An examination by The New York Times found that gunmen who perpetrated 16 recent mass shootings used a variety of firearms in their attacks, including handguns and assault-style weapons. While rifles like the AR-15 could increase the lethality of an attack in some situations, experts say that the use of high capacity magazines that can hold dozens of rounds of ammunition may give shooters an even greater advantage, allowing for more bullets to be fired without reloading without pause — breaks that can provide an opportunity for an officer or civilian to interrupt an attack.

Mass shooters often fit a psychological and behavioral profile.

America’s most notorious mass shooters have been young, angry men who displayed antisocial behavior before they carried out attacks. Many perpetrators of mass killing also have a history of domestic violence; social scientists have found that the same factors drive the two phenomena.

A 2015 Huffington Post analysis looked at incidents over a five year period in which at least four people were killed with a gun, including shootings in domestic settings (a criteria which the FBI’s definition excludes). A majority of the shootings involved a family member or intimate partner — women and children comprised 64 percent of victims.

Media coverage of the mentally ill exaggerates their role in gun violence.

While mental illness may drive some mass shooters to kill, media coverage of the mentally ill exaggerates their role in gun violence. Less than 4 percent of violent acts are carried out by someone who is mentally ill — and research shows that individuals with mental illness are a greater risk to themselves.

Violent behavior and substance is a better predictor of future violence than a mental health diagnosis. The substance most often associated with violent crimes is alcohol.

Mass shooters in America often target workplaces and schools.

A University of Alabama researcher found that a uniquely American cultural and mental strain leads mass shooters to target workplaces and schools — as opposed to the military installations often targeted by international mass shooters — because these institutions represent the social systems that the gunmen believe mistreated them.

Most mass shooting victims are black.

The gun violence burden is disproportionately carried by men of color, who comprise half of American gun death victims — despite making up just 6 percent of the population. A New York Times analysis of shootings that killed or wounded four or more people in 2015 found that two-thirds of the victims were black. Seventy-two percent of the victims were men.

Some urban neighborhoods are plagued by persistent, truly epidemic shooting rates. Violent gun crime varies even more within American cities than between them. A glance at murder rates by neighborhood — not just by city — reveals a terrible murder inequality that ensnaring men of color in a cycle of killing.

It’s rare for a “good guy with a gun” to intervene during an active shooting.

Gun rights activists say they want to abolish so-called gun-free zones — areas where guns are not permitted, including schools and many private businesses — because they deny civilians carrying concealed handguns the opportunity to stop a massacre, while providing an unprotected target for mass shooters looking to perpetrate large-scale carnage.

Indeed, the foundational tenet of the National Rifle Association’s agenda is that more “good guys” carrying guns in public will reduce crime and make society safer. There is no evidence to support this claim. Of the 160 active-shooting incidents from 2000 to 2013 that were analyzed by the FBI in 2014, only one active shooting was stopped by a concealed-carry license holder. Twenty-one were stopped by unarmed civilians.

See if a mass shooting has happened near your home.

To find out how many mass shootings have happened near you, type an address here to see how much gun violence has touched your neighborhood between June 30, 2015 and June 30, 2016. If you spot a cluster of four or more fatalities, that counts as a mass shooting, according the FBI.