Police officers are most likely to be killed in states where the most people own guns, a new study finds.
The report is sure to be controversial, but it adds a new dimension to a conversation that's recently been focused more on police shootings of unarmed Americans.
This study looks at who's killing the cops, and it's overwhelmingly people with private guns, David Swedler of the University of Illinois at Chicago School of Public Health found.
"If we're interested in protecting police officers, we need to look at what's killing them, and what's killing them is guns," says Swedler.
Swedler's team used the FBI's Uniformed Crime Reporting database to check on all homicides of law enforcement officers between 1996 and 2010. They used a giant survey called the Behavioral Risk Factor Surveillance System to get data on gun ownership.
Their findings: The No. 1 cause of on-the-job death for police is motor vehicle accidents. But gun deaths came second, they reported in the American Journal of Public Health.
"We were not surprised to find that firearm ownership is associated with homicide rates," Swedler told NBC news.
"The big surprise finding to me was the differences in homicide rates among officers in states with the lowest gun ownership compared to states with highest gun ownership," he added.
"Officers are three times more likely to be murdered on the job in high gun ownership states in comparison with low gun ownership states. That was the big wow for me."
Police and sheriffs' organizations disagree on gun laws, gun control and gun ownership and whether limits would help reduce crime.
Swerdler's one of a group of researchers who want to see what the data shows, and who consider gun ownership and gun laws matters that can legitimately be studied and debated as public and occupational health issues.
"To me, an officer is a worker just like any worker in America. Workers have the right to come home from work at the end of the day," Swedler said.
"We in occupational health and safety look to protect the lives of workers."
His team found 782 homicides of police officers over the 15 years they looked. Of those, 716 were committed using guns and 515 of those with handguns.
On average, 38 percent of U.S. households have at least one gun, ranging from 4.8 percent of homes Washington, D.C. to 62 percent in Wyoming, the researchers found. This fits in with other studies.
Alabama, Alaska, Arkansas, Mississippi and Montana were the states with the highest rates of both gun ownership and for law enforcement killings. Connecticut, Massachusetts, New Jersey, New York and Rhode Island had the lowest per-capita rate for both.
It's important to calculate the rates by per capita, said Swedler, because large states have more killings and more guns simply because they have more people.
Swedler said his team also made sure that police killings were not related to violent crime in general.
"Hypothetically, officers might be put at increased risk if they are more frequently encountering violent criminals, but our data doesn't find that to be the case," he said. "We find that officers are at an increased risk for being killed the more frequently they encounter guns in public settings."
One big driver of this: domestic violence.
"Research shows that responding to domestic violence calls are one of the most common situations in which officers are killed," Swedler said.
"In states where firearms are more prevalent, officers responding to reports of domestic violence are more often entering potentially lethal situations compared to officers responding to such calls in states with lower firearm prevalence," Swedler said
Police need to understand this, Swedler said, and police department leaders should consider rates of gun ownership in training officers.
"How can we prepare our officers in light of the presence of guns in our state?" Swedler said.
And voters need to decide what to do about findings like his, he said.
"If people in the United States are concerned about the lives of police officers, think about the laws in your state regarding firearms," Swedler said.
"We didn't say one type of law is going to sentence officers to death, whereas another type of law is going to save all officers' lives. What we are saying is consider laws in your state about firearm possession."
But Everytown, an organization that groups Mayors Against illegal Guns, Moms Demand Action for Gun Sense in America and other advocates together, says its research shows that just enforcing existing gun laws can help.
"Original research Everytown recently published with the Major Cities Chiefs shows that more than half of law enforcement shot to death were killed by criminals who were barred by law from buying or owning guns but got them anyway," Ted Alcorn, research director for the group, said via email.
"Nothing does more to reduce these deaths than a strong background check system: FBI data shows that in states that require background checks for all handgun sale, blocking criminals from buying guns in unlicensed sales online or at gun shows, there are 48 percent fewer law enforcement officers killed with handguns," he said.