Mass shootings have propelled debates about U.S. gun laws, but they haven't led to much in the way of research. In large part, that's due to the lobbying efforts of the gun industry, according to the Associated Press.

Thirty years ago, health researchers began to seriously investigate gun violence as firearm homicide rates reached epidemic levels, but the field was upended by organizations like the National Rifle Association, which pressured politicians to defund research that might affect its interests by recommending limitations on gun ownership.

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"So funding for basic gun violence research and data collection remains minuscule—the annual sum total for all gun violence research projects appears to be well under $5 million," the AP reported. "A grant for a single study in areas like autism, cancer, or HIV can be more than twice that much."

What we do know is compelling, but research remains incomplete.

We know, for example, that there is a gun for every man, woman, and child in America; we know that this country accounts for one-third of mass shootings globally despite having only five percent of the world's population; and we know that guns are one of the top five killers of people ages 1 to 64, according to the Centers for Disease Control and Prevention.

But researchers have been largely blocked from expanding our knowledge of guns and how they relate to gun-related suicide rates, firearm murders, and mass shootings.

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"The CDC, the federal government's lead agency for the detection and prevention of health threats, took an early leading role in fostering more research into violence," the AP reported. "But beginning in the 1980s, the National Rifle Association tried to discredit CDC-funded studies, accusing the agency and the researchers the agency funded of incompetence and falsifying data."

In 1996, lawmakers who were friendly with the NRA earmarked $2.6 million the government had set aside for CDC research into firearm injury for "traumatic brain injury" research, changing the language of the budget so that the agency could not receive federal funding for studies that "might be used, in whole or in part, to advocate or promote gun control."

And now there's an increased reluctance to put forward new research proposals, leaving research institutions with limited resources to fund independent studies. Some researchers have said that they fear the political repercussions of advancing gun studies because they've received death threats for their work from pro-gun supporters.

"If the climate was right and the funding was there, it would make sense to focus on gun violence prevention," Dr. Michael Levas, a researcher in Milwaukee, Wisconsin, told the AP. "But right now, it would be a dead end."