Three people have been killed and at least 140 injured in two explosions at the finish line of the Boston Marathon, police say. TV footage showed bloodied runners and spectators being treated at the scene and the road strewn with debris. Boston police said a third incident occurred at the city's JFK Library and was under investigation. In a TV address, President Barack Obama said those responsible would feel the "full weight of justice". He said the authorities did not know who carried out the attack or why.

The Boston Marathon

- World's oldest annual marathon - 2013 was the 117th event
- Second biggest single-day sporting event in the US behind the Super Bowl, in terms of media coverage
- Organised by the Boston Athletic Association, the course is 26.2 miles
- This year there were 28,000 runners, representing athletes from every US state and more than 90 countries
- More than 500,000 spectators line the course every year

source: http://www.bbc.co.uk/news/world-us-canada-22160691