The United States and other countries increase security in response to deadly bombings in Boston

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.

SWAT officers patrol the Copley Square area after explosions near the finish line of the Boston Marathon in Boston, Massachusetts April 15, 2013.

A sign is displayed by the road side warning of a British police security operation taking place along the forthcoming London Marathon route in London.

Tourists take pictures near a German police car guarding in front of the United States embassy in Berlin.

Federal Bureau of Investigation (FBI) agents arrive at the scene after explosions near the finish line of the Boston Marathon in Boston, Massachusetts April 15, 2013.

Backdropped by Buckingham Palace, Queen's Life Guards and police offcers ride down the Mall in London where the finish area of the forthcoming London Marathon.

Members of the U.S. Army National Guard Joint Task Force Empire Shield patrol Grand Central Terminal in New York, April 16, 2013.

Boston police officers keep a perimeter secure in Boston's Copley Square.