New York Police Department officers investigate a commuter's bag in the Times Square subway station in New York during an increase in security following fatal explosions in Boston, April 15, 2013. Two bombs ripped through the crowd at the finish line of the Boston Marathon on Monday, killing two people and injuring dozens in what a White House official said would be handled as an "act of terror." [Photo/Agencies]

New York Police Department officers stand in the Times Square district of New York underneath a news ticker displaying details from fatal explosions in Boston, April 15, 2013. Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons. [Photo/Agencies]