The vast majority of guns used in 15 recent mass shootings, including at least two of the guns used in the San Bernardino attack, were bought legally and with a federal background check. At least eight gunmen had criminal histories or documented mental health problems that did not prevent them from obtaining their weapons. - DEC. 2, 2015: Syed Rizwan Farook and Tashfeen Malik, husband and wife, killed 14 people at a holiday office party in San Bernardino, Calif. Four... (GunPolicy.org)