marathon

Competitors run in the Badwater Ultramarathon in Death Valley National Park

Lucy Nicholson/Reuters

Competitors take part in the Badwater Ultramarathon in Death Valley National Park, California, in temperatures that can reach 55 degrees centigrade

US President Barack Obama and First Lady

Jewel Samad/AFP

President Barack Obama heads to Boston to mourn victims of the Boston Marathon attacks. No arrests have been made in connection with Monday’s twin bombings near the finish line of the race, which sent metal fragments and nails into a crowd of runners and spectators, killing three people and wounding 180.

Miller, a Transit Police dog with the Massachusetts Transportation Authority Explosives Detection Unit, watches as commuters enter the subway system at Back Bay Station a day after two explosions near the finish line of the Boston Marathon in Boston, Massachusetts April 16, 2013. Two bombs packed with ball bearings tore through crowds near the finish line of the Boston Marathon on Monday, killing three people and triggering  a massive hunt for those behind an attack the White House said would be treated as "an act of terror."

Jessica Rinaldi/Reuters

Miller, a Transit Police dog, watches commuters at Back Bay Station a day after two explosions in Boston. Miller, a Transit Police dog with the Massachusetts Transportation Authority Explosives Detection Unit, watches as commuters enter the subway system at Back Bay Station a day after two explosions near the finish line of the Boston Marathon in Boston, Massachusetts April 16, 2013. 

Bill Iffrig, 78, lies on the ground as police officers react to a second explosion at the finish line of the Boston Marathon in Boston, Monday, April 15, 2013. Iffrig, of Lake Stevens, Wash., was running his third Boston Marathon and near the finish line when he was knocked down by one of two bomb blasts.

John Tlumacki/Boston Globe/AP

Bill Iffrig, 78, lies on the ground as police officers react to a second explosion at the finish line of the Boston Marathon on April 15 2013. Mr Iffrig, of Lake Stevens, Washington, was running his third Boston marathon and was near the finish line when he was knocked down by one of two bomb blasts.