More than a decade before a San Jose, California, transit employee killed nine co-workers on May 26, an ex-girlfriend accused him in court of physical abuse and sexual assault. The case did not lead to a criminal conviction or a permanent restraining order, which would have barred him from buying guns.

The woman told a local news station that the man had always been mentally unstable and prone to bouts of rage. But with this deadly public rampage, his abuse crossed a line into the public realm. “He’s a murderer,” she said. “He killed innocent people.”

The San Jose gunman is far from an anomaly. Perpetrators of some of the country’s deadliest shootings have had domestic violence charges, incidents, or allegations in their backgrounds. The man who killed 49 people at Pulse nightclub in Orlando in 2016 was abusive toward his ex-wife, who described frequent beatings. Five years before an Air Force veteran killed 26 people at a church in Sutherland Springs, Texas, in 2017, he was court-martialed for attacking his then-wife and her infant daughter. The teenage perpetrator of the 2018 shooting at Marjory Stoneman Douglas High School in Parkland, Florida, was reportedly abusive toward his ex.

A new study released late last month further solidifies the connection between domestic violence and a propensity for future, public acts of violence. Researchers from the Educational Fund to Stop Gun Violence and Johns Hopkins analyzed 110 gun murders of four or more people between 2014 and 2019 and found that in 68 percent of incidents, the perpetrator either killed an intimate partner or a family member, or had a history of domestic violence. 

When that’s the case, more people tend to die. The study, published in Injury Epidemiology, also found that mass shootings in which gunmen target an intimate partner or family member have a fatality rate that’s just over 20 percentage points higher than other mass shootings. That could be because the victims are targeted by perpetrators with a clear intent, the researchers said, unlike gunmen who fire indiscriminately in a public place.

Understanding how rage can boil over into violence — and removing guns from people most likely to use them in acts of violence — should be a public policy imperative, the study’s lead author told The Trace. “We know that past violence is the best predictor of future violence,” said Lisa Geller, state affairs manager at Educational Fund to Stop Gun Violence, “and often that past violence is domestic violence.”

Some mass shootings began as attacks on romantic partners or family members that spilled over into the public realm; some transpired in private and involved only intimate partners and their children. The Gun Violence Archive reports that at least 190 mass shootings (defined as four or more people injured or killed) since 2013 have been domestic in nature. All of them demonstrate that gun violence is often a lot more intimate than the way mass shootings are portrayed in the media: as random, public attacks perpetrated by strangers. In reality, Americans are still most likely to be shot by someone they know. 

Here, we break down all the ways domestic and mass shootings intersect — and what can be done to break the connection:

Some perpetrators of active shootings have a history of domestic violence

This history can show up in police reports, arrests, dropped charges, convictions, or restraining orders. April Zeoli, a professor of criminal justice at Michigan State University who researches intimate partner homicide, said both intimate partner abuse and mass public violence share the same root cause. “Domestic violence is about power and control over your intimate partner,” she told The Trace. “And a mass shooting is about control over whether people live or die, so it is that ultimate power.”

After the Pulse shooting, Gene Deisinger, a psychologist who handles threat assessment for Virginia Tech’s campus police department, told The Trace that he keeps an eye out for people who have committed acts of domestic violence or other expressions of “acute anger.”

“Once you’ve crossed the threshold of being angry and violent, it’s easier to cross again in the future,” Deisinger said.

That was the case in 2019, when an Illinois man killed six co-workers at a warehouse in Aurora years after an ex-girlfriend filed two restraining orders against him. The perpetrator of the June 2017 Congressional baseball shooting, which left four people wounded, including Representative Steve Scalise, had been arrested 11 years earlier for hitting and choking his daughter and shooting at her friend’s boyfriend. Less than two hours before a Kansas man shot 14 co-workers at a lawnmower factory in 2016, killing three of them, he was served with a restraining order by a former domestic partner. 

Public mass shootings are sometimes preceded by a domestic shooting

Scores of domestic shootings have progressed to public gun rampages, moving from intimate targets to unknown ones in a matter of hours. In 2019, a California man went on a 12-hour shooting spree across the San Fernando Valley, first killing his father and his brother and wounding his mother at their home in Canoga Park, then traveling to North Hollywood and gunning down an acquaintance and a passenger on a bus. The night before a California man embarked on a 2017 shooting spree through the rural community of Rancho Tehama Reserve that left four people dead and 11 others wounded, he fatally shot his wife and hid her body under the floor of their trailer. 

Even the first modern mass shooting began with domestic homicide: The perpetrator of the 1966 shooting at the University of Texas at Austin killed his wife and mother the night before his rampage.

Zeoli said it’s been difficult to explore the psychology of mass shooters who also kill family members because so many of them kill themselves as well. But she suspects that suicide is a driving factor. “We know that suicidality of a violent partner is one of the predictors of intimate partner homicide,” she said. “So it could be that whatever the impetus for suicide is, that’s driving this larger public mass shooting as well.”

Some domestic shootings are carried out in public and leave bystanders dead, too

In recent years, several shooters targeting intimate partners in public places also killed their victim’s family members or co-workers, or strangers who happened to be nearby.

In November 2018, a man gunned down his ex-fiancée, emergency room doctor Tamara O’Neal, outside the Chicago hospital where she worked following an argument over their broken engagement. He then ran inside the hospital and fatally shot a 25-year-old pharmacy resident and a responding police officer. In October 2012, a Wisconsin man walked into the spa where his ex-wife worked and shot seven people, killing three of them, including his ex-wife, Zina Haughton. Two weeks earlier, Haughton had filed a restraining order against him detailing abuse and death threats.

Zeoli said that while “the vast majority” of domestic violence perpetrators never become mass shooters, it’s easy to imagine how abusers can perpetrate violence outside the home. “People who commit domestic violence have demonstrated that they are more than willing to terrorize and harm the people they purport to love,” she said. “It may be that whatever psychology underlies their desire to harm, like seriously harm and inflict damage on the people they love, is the same psychology that lends itself to mass shootings.”

Then there are domestic mass shootings, which target family members and take place at home

According to one estimate, the majority of mass gun murders in the U.S. take place in private, and they’re usually domestic. So-called family annihilators kill their entire immediate families and often themselves, most often with guns. Because there’s no danger to the public, many of these stories go unreported by most national media outlets.

In February, a man in Muskogee, Oklahoma, was charged with killing his brother and five relatives under 10, three of whom were his children.

“We think about mass shootings in public places, and people are scared to go to the grocery store, they’re scared to go to concerts,” said Geller, the author of the Injury Epidemiology study. “But there are so many people who are scared to be in their own homes, and that often gets overlooked.”

Mass domestic killings that transpire at home can also kill people who are not related to the gunman or victim. On Mother’s Day, a Colorado man fatally shot his girlfriend and five of her family members at a birthday party in a mobile home park in Colorado Springs. 

What can be done to keep firearms from domestic abusers?

The presence of guns can turn domestic disputes deadly. Abused women are five times more likely to die if their abuser has access to a gun. If their partner threatened them with a gun, the risk of death is 20 times higher

But keeping firearms away from abusers is a challenge, made harder by loopholes that allow people to arrange gun sales online, at gun shows, and from private sellers without a background check. Two days before Zina Haughton was gunned down at her workplace, her husband was stripped of his gun rights because of a restraining order. He turned to the online gun broker Armslist to purchase the handgun he used.

Geller recommends the nationwide adoption of extreme risk protection orders, or red flag laws, which allow police or family members to petition a judge to disarm potentially dangerous people. Nineteen states and Washington, D.C., have enacted red flag laws, most of them since the massacre at Marjory Stoneman Douglas High School in Parkland, Florida in 2018. ERPOs are civil protection orders, which means they’re not accompanied by criminal charges. “The goal is to prevent acts before they occur,” Geller said. “There are certain periods of time where some individuals should not have a gun. These are not permanent bans on firearm ownership.” In 2019, a team from UC Davis School of Medicine analyzed court records from 159 extreme risk protection order cases in California between 2016 and 2018 and identified 21 cases in which one was used to try to prevent a mass shooting.

There are also state-level gun bans for domestic abusers. A 1996 federal law called the Lautenberg Amendment bans gun possession for people convicted of misdemeanor domestic violence and those who have been served with permanent orders of protection. But it has significant loopholes: It doesn’t apply to partners who are dating, only ones who have lived together, been married, or have a child in common, an omission known as the “boyfriend loophole.” Nor does it extend to people convicted of misdemeanor stalking. The vast majority of domestic violence occurs among dating partners.  

And the law can be difficult to enforce, especially when it comes to abusers who own guns before they are convicted. The federal government doesn’t require abusers to relinquish their weapon; it relies on the honor system (though federal prosecutors in a handful of states are cracking down on domestic abusers who unlawfully keep guns). 

Twenty-nine states have crafted matching laws that empower local authorities to enforce Lautenberg. States that go beyond federal law and require abusers to relinquish their weapons to police have lower rates of intimate partner homicide, a 2017 study found. And 28 states and D.C. prohibit some abusive dating partners from gaining access to firearms, effectively closing the boyfriend loophole. 

Without federal uniformity, cases slip through the cracks, and lives can be lost. Earlier this year, my colleague Ann Givens reported on Rosemarie Reilly, a Michigan woman killed by her ex-boyfriend in 2016. Three weeks before she was murdered, Reilly had gotten a protective order against him, but a judge denied the gun restriction.