Tashfeen Malik (left) pledged allegiance to the Islamic State on Facebook just before she and her husband, Syed Farook, went on a shooting spree that left 14 dead in San Bernardino, Calif., last December. The couple is shown here in a 2014 photo. In just over a year, three separate attackers have declared support for ISIS as they carried out shootings in the U.S. (AP)