High-profile, multiple-casualty attacks within the United States are the holy grail for some terrorist organisations. Yet they have been mercifully rare since the al-Qaeda airliner attack in 2001 that killed 2992 people.
In response, US President George W. Bush announced a war on terrorism that led to the invasions of Afghanistan and Iraq.
Congress passed the Homeland Security Act 2002 — overhauling security arrangements in the largest restructuring of the US government in recent history. It also passed the US Patriot Act, which civil liberty groups say allows law enforcement to invade the privacy of citizens and eliminates judicial oversight.
Criticisms of the response are many and varied, but they have been successful in preventing terror attacks on US soil.
There have, however, been a small number of attacks and attempted attacks in recent years — leading up to the awful news today of bomb blasts at the Boston Marathon that have killed two and injured more than 50.
■ On June 1, 2009 a Muslim convert from Tennessee shot two soldiers outside a military recruiting centre in Arkansas, killing one. He pleaded guilty and claimed ties to al-Qaeda.
■ On December 25, 2009 a Nigerian man on a flight from Amsterdam to Detroit failed to ignite an explosive device hidden in his underwear. He later said he was directed by al-Qaeda.
■ New York’s Times Square was evacuated on May 1, 2010 when a car bomb was ignited but failed to detonate. Faisal Shahzad pleaded guilty to placing the bomb and 10 terrorism and weapons charges.
■ Nine days later, in Jacksonville Florida, a pipe bomb exploded while 60 Muslims were praying in a mosque. No one was injured.
■ On October 29, 2010 two bombs were found in packages on separate cargo planes en route from Yemen to the United States.
■ On January 17, 2011 a pipe bomb was discovered along the route of the Martin Luther King Jr. memorial march. The “viable device”, set up to spray marchers with shrapnel and cause multiple casualties, was defused.