The exterior of Annie's Get Your Gun store is seen where Syed Rizwan Farook is reported to have bought a gun used in the attack at the Inland Regional Center in San Bernardino on December 5, 2015 in Corona, California. Police continue to investigate the mass shooting at the Inland Regional Center in San Bernardino that left 14 people dead and another 17 injured on Dec. 2nd. Joe Raedle/Getty Images

Seventeen years after his father was shot and killed in a robbery attempt, Eduardo Franco, who is from Brazil, found a way to shed light on fatalities and injuries in another country with a long history of gun violence: the United States. True, he is an outsider on the topic. Franco is a cancer epidemiologist, not an expert in firearms and public health.

Though he spent five years in the U.S., he now lives and works in Montreal where he is a professor and chairman of the oncology department at McGill University. But in 2013 Franco became editor-in-chief of the journal Preventive Medicine, giving him a “bully pulpit,” he says.

Researchers have long tackled health consequences associated with tobacco, asbestos and automobile safety. With guns, however, the body of academic research on death and injury prevention is slim. It's so scarce, in fact, that when Franco dedicated an entire issue to the subject in October, he estimated that the mere 10 published papers would account a significant portion of this year’s output on guns in medical and public health literature.

“I consider this the last low-hanging fruit in all of public health,” Franco said.

With each tragic mass shooting like the one in San Bernardino, California, on Wednesday, Americans search for answers to basic questions about guns. Just how many guns are there in the U.S.? How many are sold each year? Are people more likely to die from a gun in areas where ownership is high?

Guns kill more than 32,000 people a year in the U.S. and injure another 67,000, according to an analysis published in the October issue of Preventive Medicine. Firearm suicides and firearm homicides are among the leading causes of injury deaths in the U.S., according to the Centers for Disease Control and Prevention (CDC). Yet, there is much that public health researchers don’t know about firearms.

By law, the federal government cannot register and track gun purchases. A dearth of financial support for research is also part of the reason little data exists, and many institutions and medical experts fear that pursuing the topic will attract controversy. But as a result, experts say they lack the data they need to make recommendations that might reduce injuries and prevent deaths.

“How do we combat an epidemic?” asksed Bindu Kalesan, a professor at Boston University’s School of Medicine. “We do case control studies to see: What is the culprit? Where is the first case? Then we combat it. That is public health.”

Employees and other people are evacuated by bus from the site of a mass shooting Dec. 2, 2015, in San Bernardino, California. David McNew/Getty Images

Kalesan’s research includes how firearm fatality rates differ by state, and by race, as well as how hospitalization rates of children with gunshot injuries vary by race, ethnicity and income. She is very interested in studying gun sales patterns -- which guns are used in crimes, where they were bought and who owned them along the way. To do that, Kalesan would need a control group -- a dataset of guns that haven’t been used in crimes -- so she can identify the factors that make a particular gun more likely to be used in committing a crime. The data she seeks is unattainable. The Firearm Owners Protection Act of 1986 forbids the creation of a national registry of gun sales.

“My science is about comparisons,” Kalesan said. “If you do not have the right comparison, my science kind of dies.”

Jon Vernick, co-director of the Johns Hopkins Center for Gun Policy and Research, tells his students that our best estimate of how many guns are in private hands in the U.S., about 300 million, is just that -- an estimate. It comes from telephone survey data.

The closest proxy for sales data is FBI background checks run on purchases conducted at federally licensed gun stores, but Vernick said “our best estimate” is that about 40 percent of gun transactions involve private sales, for which no background check is required. Thirty-two states don’t extend background checks into any private sales. Ten states and the District of Columbia have a licensing system.

“It’s a problem that we don’t know with precision how many guns are sold, and it’s a problem that we don’t know about America’s gun stock with greater precision,” Vernick said. Data from the Bureau of Alcohol, Tobacco, Firearms and Explosives provides some insights into how many handguns and long guns are manufactured in the U.S., and exported and imported each year. However, “we don’t know, for example, how many are AR-15s,” he said, or how many handguns are good quality “versus how many are poorly made, inexpensive guns.”

How would such data be useful? Vernick draws a comparison to the data the federal government maintains on car safety and accidents, including analysis of crash scenes. Per mile driven, the rate of motor vehicle fatalities has decreased more than 75 percent since the 1960s, Vernick says. “One of the reasons we’ve been able to do that is through the kind of research that tells us what’s risky, and what’s not, and what kind of interventions reduce that risk, and which don’t.”

By contrast, “we can’t figure out with precision, which guns are riskier than others,” Vernick says.

Guns are piled on the ground during the destruction of approximately 3,400 guns and other weapons at the Los Angeles County Sheriff's 22nd annual gun melt at Gerdau Steel Mill, July 6, 2015, in Rancho Cucamonga, California. California law requires the destruction of confiscated weapons. David McNew/Getty Images

Funding for public health research into firearm injuries took a hit in 1996, when lawmakers and the National Rifle Association (NRA) clamored for an end to federally backed studies that could advocate for gun control. That year, Congress sent a powerful message to researchers when it instructed the CDC it could not use funds to "advocate or promote gun control." The CDC has barely touched the topic of gun violence since, and researchers said the move had a widespread, chilling effect.

Eduardo Franco said fear has held back the scientific community for too long. “They are not lobbying for more interest in gun violence because, well, I’ll tell you like it is: They’re afraid. They’re afraid they’re going to be ostracized. They’re afraid the NRA is going to ridicule them.”

Recently some groups are speaking up. Hours before the mass shooting in San Bernardino Wednesday, the group Doctors for America presented a petition in Washington, demanding the effective federal funding ban be lifted.

“Since 1996, the federal government has spent $240 million a year on traffic safety research, which has saved 360,000 lives since 1970,” the group said in a statement joined by more than 2,000 physicians. “During that same period, there has been almost no publicly funded research on gun violence, which kills the same number of people every year.”

The dearth of government funding is not the only money problem. “Almost all major foundations are afraid to take this up because they don’t want the hassle,” said David Hemenway, director of the Harvard Injury Control Research Center.

It’s also hard to piggyback on work being done by other organizations. “Gun questions are hard to put on other people’s surveys because they’re afraid to put them on,” he said. “They’re afraid they’ll get bad feedback.” The CDC used to include several questions in its Behavioral Risk Factor Surveillance System survey about whether the respondent had a gun in the house, and how the gun was stored. It stopped including those questions in 2004, Hemenway said.

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Hemenway is a leading researcher on gun deaths among children, and he was one of the guest editors for the special issue of Preventive Medicine. He finds it difficult to encourage doctoral students to follow in his footsteps. “You don’t want to go out into the field and be a gun researcher because you’re not going to be able to make ends meet.”

An important advancement experts point to is the creation of the National Violent Death Reporting System (NVDRS). It captures details about a person’s death from multiple sources, including a medical examiner’s narrative, and, oftentimes, crime lab data about the make and model of the gun involved.

This additional information can make a big difference in how a death is categorized and therefore in how researchers and policymakers might formulate a response to it. Hemenway said children’s firearm fatalities are often misclassified as homicides when many are, in fact, unintentional shootings that could possibly have been prevented. His research shows when using the NVDRS, compared to the long-standing vital statistics system, the real number of unintentional gun fatalities among children “is about 80 percent higher.”

“It’s a wonderful data system,” Hemenway said. The NVDRS, however, isn’t employed in all states. It has gradually expanded to 32 states. The holdup? “It’s funded by Congress,” he said, “but in their wisdom, they don’t fund it all.”

Franco said in public health there is no one article that cracks the case. “No individual study solves anything, but they do add this little building block of knowledge.” Researchers who studied tobacco, asbestos, and motor vehicles all encountered pushback from industry. Franco believes opposition pushed researchers in those areas to do better.

Gun violence, he conceded, is a more complex undertaking than other public health concerns. All around the world firearms are integral to countries’ defense systems. In the U.S. in particular, he said, “it is this combination of personal rights and entrepreneurial spirit, plus that fear of intervention by Big Brother, by the government.”

It is not going to be simple. “Which,” said Franco, who dedicated the issue of Preventive Medicine in his father’s memory, “is going to require more brain power.”