President Barack Obama greets with his supporters as he leaves during the grassroots event at University of Iowa on Friday, Sept. 7, 2012., in Iowa City, Iowa.
Mark Lennihan photo
One World Trade Center, center, rises above the National September 11 Memorial and Museum at the World Trade Center, Thursday, Sept. 6, 2012 in New York. Tuesday will mark the eleventh anniversary of the terrorist attacks of Sept. 11, 2001. The World Financial Center is on the left, and Four World Trade Center is at right.