There is as yet no official confirmation on what caused the explosion, but the proximity of the blasts - which happened 50 yards and 10 seconds apart from each other - and the fact that ball bearings were found on the scene strongly suggests that this was a terrorist bombing.
Sadly, sport is no stranger to terrorism. The most recent high-profile attack came at the 2010 African Nations Cup when the Togo national team bus was attacked while on its way from Congo-Brazzaville to Cabinda in Angola.
On that occasion a Cabindan separatist group opened fire on the bus, killing Angolan driver Mario Adjoua, Togo assistant manager Abalo Amelete and the team's media officer Stanislas Ocloo, while nine others were injured.
The year before, the Sri Lankan cricket team was travelling to the Gaddafi Stadium in Lahore, Pakistan to play the third day of a Test match when gunmen opened fire - including rockets and grenades - on their bus. Six Pakistani policemen and two civilians were killed while six members of the team were injured. A number of suspects were arrested but released due to lack of evidence. Pakistan has played home matches in the United Arab Emirates ever since.
In 2008 a suicide bomber targeted a marathon race in Sri Lanka in an horrific incident that appears to have several parallels with the Boston Marathon explosions. The bomber killed 15 people and injured almost 100 attending a race to mark the start of the Sinhalese New Year. The Tamil Tigers were blamed as highways minister Jeyaraj Fernandopulle - a critic of their movement - was among the dead, along with former Olympic marathon runner KA Karunaratne and national athletics coach Lakshman de Alwis.
That same year, the Paris-Dakar rally was cancelled following threats by Al Qaeda and the murder of a family of French tourists in Mauritania. It was moved to South America the year after, and has never returned to Africa since.
Though all those incidents took place in regions where terror attacks are sadly all too common, America itself has been no stranger to sports-related terrorism: the 1996 Olympic Games in Atlanta were marred by a nail bomb attack at the swimming pool. Two people were killed and 111 were injured in the attack, which happened not during competition but at a late-night concert in the Centennial Olympic Park.
Security guard Richard Jewell, who found the bag containing three bombs under a bench, was initially falsely accused; the FBI discovered in 2003 that former US Army explosives expert Eric Rudolph had planted it. Rudolph said "the conception and purpose of the so-called Olympic movement is to promote the values of global socialism" and that he had hoped to "confound, anger and embarrass the Washington government in the eyes of the world for its abominable sanctioning of abortion on demand". He was given four life sentences without the possibility of parole.
The Olympics were also targeted 24 years before that in perhaps the most notorious act of sporting terrorism ever witnessed: the 1972 Munich Games were rocked when 11 members of the Israeli team were taken hostage and eventually killed during the Games. The Black September group perpetrated the attack.
Europe has also been hit: in 2002 in Spain, Basque separatist group ETA detonated a car bomb close to Real Madrid's Estadio Santiago Bernabeu hours before the Champions League semi-final Clasico against hated rivals Barcelona - a response to the arrest of 11 members of Batasuna, a political party linked to ETA. Real - as that word, 'Royal' in English, implies - have long been considered the team of the aristocracy in Spain, and it was suggested that this was why the bomb was targeted there, though this has never been confirmed.
- Act Of Terror
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