Pedestrians walk past the Willis Tower, Tuesday, Aug. 23, 2011, in Chicago. At Willis Tower, like other skyscrapers around the country, much has changed since two hijacked jets slammed into the twin ... more 
Pedestrians walk past the Willis Tower, Tuesday, Aug. 23, 2011, in Chicago. At Willis Tower, like other skyscrapers around the country, much has changed since two hijacked jets slammed into the twin towers of New York’s World Trade Center a decade ago: Concrete barriers ring the outside of North America's tallest building, metal detectors greet visitors and sophisticated security cameras trace every movement in and around the structure. Experts say the obvious precautions still leave thousands of buildings vulnerable because the costs to retrofit existing structures may be too costly and cities and states may be slow to adopt newer, tougher building codes for new construction like those recommended after the deadliest terror attack on U.S. soil. (AP Photo/M. Spencer Green) less 
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Associated Press | Photo By M. Spencer Green
Tue, Aug 23, 2011 5:41 PM EDT