The weapons used in this week's massacre in San Bernardino, California, were purchased legally, raising questions about how preventable gun violence is under current U.S. firearm laws.
Eighty-two percent of weapons involved in mass shootings over the last three decades have been bought legally, according to a database compiled by Mother Jones magazine that defines a mass shooting as taking the lives of at least four people in a public place. Using that criteria, Mother Jones found 73 mass shootings since 1982.