The Sept. 11 attacks were an anomaly in an overall gradual decline in the number of terrorist attacks since the 1970s, according to the Global Terrorism Database. Since 2001, the number of fatalities in terrorist attacks has reached double digits in only one year, 2009, when an Army psychiatrist killed 13 people at Fort Hood, Tex., officials say. That was a sharp contrast with the 1970s, by far the most violent decade since the tracking began in 1970. “People are actually surprised when they learn that there’s been a steady decline in terrorist attacks in the U.S. since 1970,” http://www.nytimes.com/2013/04/17/us/bombings-end-decade-without-terror-in-us.html?ref=us About half of the attacks worldwide, and nearly a third of those in the United States, have never been solved. In the 1970s, about 1,350 attacks were carried out by a long list of radical groups, including extremists of the left and the right, white supremacists, Puerto Rican nationalists and black militants. The numbers fell in the 1980s, as the groups were eroded by arrests and defections, and again in the 1990s, after the collapse of the Soviet Union, which had inspired or covertly supported some violent leftist groups. There were about 40 percent more attacks in the United States in the decade before Sept. 11 than in the decade after.
Spectators at the Boston Marathon described a heavy security presence, as has become standard at public events since 2001, including bomb-sniffing dogs that were deployed before the race.