The study, which is set to be published in the Journal of Urban Health, estimates that the law's removal was responsible for an extra 55 to 63 murders per year, between 2008 and 2012.
Prior to 2008, Missourians in the market for a handgun had to be screened by the local sheriff and apply for a license. But the permit-to-purchase law was repealed in 2007, much to the delight of the National Rifle Association, which advocated for the law's removal.
Researchers say the connection between the revoked law and increased gun violence is clear. All imaginable variables were controlled for, they say, including changes and trends in policing levels and crime rates, as well as rises in unemployment and poverty.
"This study is compelling confirmation that weaknesses in firearm laws lead to deaths from gun violence," Professor Daniel Webster, director of the research and policy center, told the BBC. "That upward trajectory did not happen with homicides that did not involve guns; it did not occur to any neighboring state; the national trend was doing the opposite."
Professor Webster was in Chicago speaking about the study this week at the annual meeting of the American Association for the Advancement of Science.