An additional 55 to 63 people have been murdered each year in Missouri following the state’s 2007 repeal of its permit-to-purchase (PTP) handgun law, which required all handgun purchasers to obtain a license verifying that they have passed a background check.

According to a new study from the Johns Hopkins Center for Gun Policy and Research, to be published in an upcoming issue of Journal of Urban Health, the increase in murders with firearms in Missouri began in the first full year after the PTP handgun law was repealed. Data from crime gun traces revealed simultaneous large increases in the number of guns diverted to criminals and in guns purchased in Missouri that were subsequently recovered by police in border states that still had PTP laws, Science Daily reports.

The researchers found that the spike in murders only occurred for murders in Missouri committed with a firearm, and was widespread across the state’s counties. They also analyzed annual data from death certificates through 2010 compiled by the Centers for Disease Control and Prevention, and determined that the repeal of Missouri’s PTP law was associated with a 23-percent increase in firearm homicide rates.