London Olympics Security

A police inflatable rib performs during a combined Police and Royal Marines security exercise on the River Thames in London, Thursday, Jan. 19, 2012. The security exercise was performed in preparation for the London 2012 Olympic games. (AP Photo/Kirsty Wigglesworth)

A police inflatable rib performs during a combined Police and Royal Marines security exercise on the River Thames in London, Thursday, Jan. 19, 2012. The security exercise was performed in preparation for the London 2012 Olympic games. In the background at river level, left, is the Thames Barrier, and right the North Greenwich Arena, originally built for the Millennium celebrations and known as the Millennium Dome, and the skyline of the Docklands financial district. (AP Photo/Kirsty Wigglesworth)

A Lynx helicopter hovers over a commuter boat during a combined British police and British Royal Marines security exercise on the River Thames, London, Thursday, Jan. 19, 2012. British police and military forces using a helicopter and inflatable boats teamed up Thursday to practice security procedures designed to protect the Thames river from possible terrorist attack during the 2012 London Olympics. (AP Photo/Kirsty Wigglesworth)

Security forces take part during a combined Police and Royal Marines security exercise for the London 2012 Olympic Games on the River Thames in London, Thursday, Jan. 19, 2012. The security exercise was performed in preparation for the upcoming London 2012 Olympic games, that start in July. (AP Photo/Alastair Grant)

Security forces take part during a combined Police and Royal Marines security exercise for the London 2012 Olympic Games on the River Thames in London, Thursday, Jan. 19, 2012. The security exercise was performed in preparation for the upcoming London 2012 Olympic games, that start in July. (AP Photo/Alastair Grant)

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, Jan. 19, 2012. The security exercise was performed in preparation for the upcoming London 2012 Olympic games, that start in July. (AP Photo/Alastair Grant)

Security forces take part during a combined Police and Royal Marines security exercise for the London 2012 Olympic Games on the River Thames, backdropped by Canary Wharf in London, Thursday, Jan. 19, 2012. The security exercise was performed in preparation for the upcoming London 2012 Olympic games, that start in July. (AP Photo/Alastair Grant)

Security forces perform during a combined Police and Royal Marines security exercise on the River Thames in London, Thursday, Jan. 19, 2012. The security exercise was performed in preparation for the London 2012 Olympic games. (AP Photo/Kirsty Wigglesworth)

Security forces perform during a combined Police and Royal Marines security exercise on the River Thames in London, Thursday, Jan. 19, 2012. The security exercise was performed in preparation for the London 2012 Olympic games. (AP Photo/Kirsty Wigglesworth)

Police and Royal Marines boats perform during a combined Police and Royal Marines security exercise on the River Thames in London, Thursday, Jan. 19, 2012. The security exercise was performed in preparation for the London 2012 Olympic games. (AP Photo/Kirsty Wigglesworth)

Security forces take part during a combined Police and Royal Marines security exercise for the London 2012 Olympic Games on the River Thames in London, Thursday, Jan. 19, 2012. The security exercise was performed in preparation for the upcoming London 2012 Olympic games, that start in July. (AP Photo/Alastair Grant)