Back Bay Station

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. Read more