In this Friday, July 1, 2011 photo a New York City police 

In this Friday, July 1, 2011 photo a New York City police officer with a bomb sniffing dog patrols at a Times Square subway station in New York. Since terrorists brought down the twin towers on Sept. 11, 2001, subways have been bombed across the world, including Madrid, London and this spring in Minsk. The possibility that New York’s sprawling, porous and famously gritty subway system could be next has become a constant worry.  (AP Photo/Mary Altaffer)
Associated Press
In this Friday, July 1, 2011 photo a New York City police officer with a bomb sniffing dog patrols at a Times Square subway station in New York. Since terrorists brought down the twin towers on Sept. 11, 2001, subways have been bombed across the world, including Madrid, London and this spring in Minsk. The possibility that New York’s sprawling, porous and famously gritty subway system could be next has become a constant worry. (AP Photo/Mary Altaffer)
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