Lessons learned from four high-casualty terrorist attacks in the United States since 9/11

Almost two decades after 9/11, the United States has suffered four high-casualty Islamist terrorist attacks: the Fort Hood shooting in November 2009, the Boston Marathon bombings in April 2013, the San Bernardino attack in December 2015, and the Orlando nightclub shooting in June 2016. These attacks killed 80 people and injured almost 400. This chapter tells the story about how the four attacks slipped through the cracks of the U.S. security apparatus and how, with the benefit of hindsight, warning signs were missed. Given the scale of the counterterrorism challenge faced by U.S. security agencies, and the difficult judgment calls they must continuously make on which probes to prioritize, the aim of this chapter is not to assign blame but to provide an overview of each case so that lessons can be learned.