It has been more than a year since white police officer Darren Wilson shot and killed unarmed black teenager Michael Brown on the streets of Ferguson, a suburb in Missouri. What began as a local incident set off a series of protests around the nation: There were Mike Browns in communities across the country and people had had enough.

The protests have continued, along with the problem: Black men are killed by police more than any other group. And, according to a new study published in Preventative Medicine, the issue is getting worse.

A team of researchers—headed by Joanna Drowos in the Medical School at Florida Atlantic University—have discovered a 45 percent increase in the number of deaths occurring during “legal intervention” between 1999 and 2013.

Mining information from a death certificate database operated by the Centers for Disease Control and Prevention, the researchers broke down deaths resulting from altercations with law enforcement by age, race, and geographical location.

They found that nearly all of those who were killed by police were male and most were between the ages of 15 and 44. As they expected, the data also showed that blacks were killed by police more often than whites—30 percent of the people with legal intervention listed on their death certificates were black. Nationally, blacks only comprise 13 percent of the total population.

The researchers also dug into geographical trends, highlighting the areas that are higher and lower risk. Drowos says this could help inform policy makers build models that work.

“By understanding these differences,”Drowos says, “it becomes possible to develop policies that can address this problem—and ultimately eliminate it.”

Still, there’s a lot more research to be done to eliminate the factors behind the trends. The researchers highlight that deaths from legal intervention fell significantly between 1979 and 1988, and remained stable afterward.

Drowos also explained that, because the data is retrospective, there’s a possibility that it was miscoded. She believes the numbers are likely underreported.

“If a death is delayed following legal intervention, it may not be identified as the cause and thus not included in this data set,” she explains. It also doesn’t include undocumented immigrants and relies on numbers that are quite small.

“Death by legal intervention happens so rarely that it is difficult to obtain reliable mortality rates (at least 20 deaths) at the county level, even in a 15 year period.” she says. “It may be more helpful to look more precisely at geographic location—such as by neighborhood—to really gain insight into high risk pockets.”

Justin Feldman, a doctoral candidate at the Harvard T.H. Chan School of Public Health agrees. He is researching the role of neighborhoods in legal intervention death rates and the factors that might lead to underreporting.

While still in the early phase of his research, he says the upward trend is surprising.

“If anything, you would expect the rates to decline because there’s been a huge reduction in violent crime since the ’90s, and violent crime and legal intervention tend to track closely with one another,” he tell Upvoted. “It is definitely worth looking more into.”

But, he also believes that the research, which looks at the issue from a systemic perspective, will help produce policy that goes deeper than just holding specific people accountable for their actions.

“A lot of the conversation revolves around bad apple officers or placing blame on individuals who are involved in altercations with police,” he explains. “Public health tries to take the blame out of the situation and to not look at only individuals but also look at the bigger social context. I hope that would open up more productive conversations where people talk about how to change policy—not just how to punish individuals, whichever side they are on.”