Obama on Boston: 'We don't yet have all the answers'
President Obama has spoken about the twin explosions at the Boston marathon and said that "we will find out who did this and hold them accountable".
Two people have been killed and at least 23 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.