US on alert amid tales of horror and heroism

US authorities are to maintain “enhanced security” at transport hubs as the nation struggles to make sense of the bomb attack on the Boston Marathon.

Three people died and more than 170 were wounded when the bombs, believed to have been filled with ball bearings and nails, detonated within 100m of each other near the finish line.

Authorities have had few details to go on. No suspects have been named publicly and it is unclear if investigators are following a definite line of inquiry. Janet Napolitano, the homeland security secretary, said there was no indication that the attacks were connected to a wider plot against the US.