Rosa Notaro, left, and Anna Sereno, right, both of Brooklyn, hold photos of their children Daniella Notaro and Arturo Angelo Sereno as friends and relatives of the victims of the Sept. 11, 2001 attacks gather for a commemoration ceremony at Zuccotti Park, adjacent to ground zero, on the ninth anniversary of the terrorist attacks on the World Trade Center, Saturday, Sept. 11, 2010, in New York. (Jason DeCrow/ASSOCIATED PRESS)
The evolution of terrorism
Does Osama bin Laden remain the greatest threat to America's security? Or have home-grown, self-radicalized, "lone wolves" become a bigger danger to the United States?
Ralph Boelter: Special Agent in Charge of the FBI's Minneapolis office.
Michael Scheuer: Former CIA Intelligence Officer.