A security exercise takes place along the River Thames on Jan. 19, 2012 in London. The exercise including around 44 police officers, 94 military personnel, 15 boats and a Royal Navy Lynx helicopter was conducted by both the Metropolitan Police and the Royal Marines and designed to test their joint capability ahead of the 2012 London Olympic Games.

A police inflatable vessel takes part during a combined Police and Royal Marines security exercise for the London 2012 Olympic Games on the River Thames in London, Thursday.

An Amtrak K-9 division police officer stands guard at Pennsylvania Station in New York on Sept. 9. The city is deploying additional resources and taking other security steps in response to a potential terror threat before the 10th anniversary of the Sept. 11 attacks.

New York City police officers search the back of a man's vehicle at a checkpoint on Broadway in New York on Sept. 9.

A detail crop of Clinton from the Situation Room picture.

President Barack Obama and Vice President Joe Biden, along with with members of the national security team, receive an update on the mission against Osama bin Laden in the Situation Room of the White House, May 1. Please note: a classified document seen in this photograph has been obscured.

A plane flies above an armored Park Police vehicle waits at the base of the Washington Monument May 2 in Washington, DC. The DC area and other places around the nation have stepped up security after it was announced that Osama bin Laden, mastermind of the September 11th terror attacks, was killed in a firefight with United States forces in Pakistan.

A New York City police patrol dog with Operation Hercules, right, barks at another patrol dog outside the police station in New York's Times Square on Monday, May 2. President Barack Obama announced Sunday night that Osama bin Laden was killed in an operation led by the United States.

A passenger is patted down by a Transportation Security Administration agent May 2 at Orlando International Airport in Orlando, Florida. Security in airports and train stations has been increased in the wake of the death of Osama bin Laden.

New York City police officers stand guard outside the Armed Forces recruitment center in New York's Times Square, Monday, May 2.

An inflatable doll drifts in the air along a beach in Cancun, Mexico on March 8. Fewer US college revelers are expected in Mexico for this year's Spring Break after Texas officials last week warned co-eds to stay away from beach destinations like Cancun.

People demonstrate against Ben Ali's Constitutional Democratic Rally (RCD) in a new wave of anger about the presence of RCD stalwarts in the transitional government, on January 20, 2011, in Tunis.

A protester gestures in front of the headquarters of the Constitutional Democratic Rally (RCD) party of ousted president Zine al-Abidine Ben Ali during a demonstration in downtown Tunis, on January 20, 2011.

Protesters stand off against a line of riot police (L) during a demonstration in downtown Tunis, on January 20, 2011.

Protesters gesture in front of the headquarters of the Constitutional Democratic Rally (RCD) party of ousted president Zine al-Abidine Ben Ali during a demonstration in downtown Tunis, on January 20, 2011.

A U.S. Secret Service agent runs along a street near where President Barack Obama is spending his holiday vacation in Kailua, Hawaii, on Thursday, Dec. 24. A man led officers on a high-speed chase that crossed a security checkpoint near the neighborhood where the president is staying. Secret Service spokesman Ed Donovan said the man was arrested a short time later and incident was not related to the president's visit, in Kailua, Hawaii.

A Secret Service agent draws her handgun on a man evading arrest by Honolulu police on Thursday.