(ILLUSTRATION: GENTLEMAN DRAUGHTSMAN)

Night after night, dressed in a black jumpsuit and a bulletproof vest, John Marsh knocks on the doors of violent felons and mentally ill people and asks them for their guns. People hand them over more often than you might expect. Last year, Marsh, a special agent with the California Bureau of Firearms, and the 33-person team he heads, confiscated 2,000 illegally-owned weapons.

Marsh is the lead agent for the Armed Prohibited Persons System, a program in which state officials comb through mountains of data to find people who have lost the right to own guns, and then send Marsh’s team to take those guns away. The agents are looking for people who bought guns legally, but were later convicted of a felony, put under a restraining order, or were deemed seriously mentally ill—any of which bar them from owning a firearm. Today the list includes almost 20,000 people who hold more than 38,000 handguns and 1,600 assault weapons.

The prohibited-persons program exists in part thanks to Garen Wintemute, a doctor and researcher at the University of California, Davis Medical Center. Wintemute has been studying gun violence since the 1980s, even sneaking into gun shows undercover to snoop on quasi-licit weapons sales. He is one of a number of academics and activists trying to get people to look at gun violence not just as a criminal-justice issue, but as a public-health one. In other words, treat gun crime as a disease, and then seek ways to stop its spread. “Whether it’s cancer or traffic accidents, you ask the same questions,” says Wintemute. “You identify the high-risk groups and then look for interventions.”

People banned from legally owning firearms can always get one on the black market. But forcing them to do that reduces the risk that they’ll use a gun at all.

In this case, one high-risk group is pistol-owning ex-cons. People with serious felony records are barred nationwide from owning guns, but research shows that such folks are nonetheless relatively likely to commit new crimes—with a gun. According to federal inmate data analyzed by researchers at Johns Hopkins Bloomberg School of Public Health, about 40 percent of felons surveyed who were convicted of gun-related offenses were prohibited from owning firearms when they committed the offense. Duke University researchers studying people charged with homicides in Illinois found similar results.

The intervention: enforce existing law by taking the guns away. Sure, people banned from legally owning firearms can always get one on the black market. But forcing them to do that apparently reduces the risk that they’ll use a gun at all. In 1990, California added people convicted of violent misdemeanors, such as resisting arrest or brandishing a weapon, to its list of prohibited persons. Wintemute’s team gathered records on two sets of people who fell into this category: individuals who bought handguns before the ban took effect, and individuals who tried to buy handguns after the ban but were turned down. Then the researchers looked at what happened over the following three years. They found that for members of both groups, the risk of committing new non-violent crimes was about identical. But in the group that was banned from buying handguns, the risk for new gun and/or violent crimes went down by about 30 percent. “That specificity of effect is a real zinger for us,” says Wintemute.

That kind of data helped persuade California lawmakers on both sides of the aisle. The bill creating the program was introduced by a Republican, and was even supported by the National Rifle Association, though that organization has since changed its position. “I’ve got nothing against getting guns out of the hands of violent criminals,” says Chuck Michel, a lawyer who represents the NRA in California. “But there are 30 different ways you can be prohibited from owning a gun. Most of the people on the list aren’t a threat, and don’t even know they’re on it.”

The program has been running quietly since 2007. In the wake of the Newtown massacre, though, media from local newspapers to the national networks have run admiring stories on it. Nationwide, Wintemute estimates that as many as 180,000 people should be on a prohibited-persons list. Still, the chance that other states will set up similar programs is slim at best. The problem isn’t just that passing gun-control measures is an uphill battle; it’s that California is essentially the only state that has the data to make the program possible.

California allows only licensed retailers to sell handguns—there’s no “gun-show loophole”—and has recorded those sales since 1996. “Only a handful of states have this data, and none of them going back as far as California,” says Wintemute. “Most have nothing at all.” And the federal government is prohibited by law from setting up a centralized gun-tracking system.

Still, the idea is picking up a little momentum. Democratic Rep. Mike Thompson of California introduced a bill in Congress to provide grants for states to set up similar programs. And California is expected to boost funding for its program to help clear the backlog and keep up with the 3,000 new names added every year.

Wintemute, meanwhile, is looking for money to evaluate what, if any, impact the program has had. “I’d expect an obvious change in the crime rate nationwide if we could disarm all the people on the prohibited-persons list,” he says. “But that would take a magic wand.” The U.S. Department of Justice has turned him down—partly because they see no point in funding research into a program other states couldn’t replicate even if they wanted to.