Stephen Knapp, a son of a victim of the 1993 World Trade Center bombing attack, wipes tears from his eyes as he participates in the 20th anniversary ceremony at Ground Zero in New York City on Feb. 26, 2013. The bomb explosion with the force of an earthquake rocked the building, killing 6 people and injuring more than 1,000. (Xinhua/Wang Lei)

Port Authority Pipes and Drums band participates in the 20th anniversary ceremony to commemorate victims of the 1993 World Trade Center bombing attack, at Ground Zero in New York City on Feb. 26, 2013. The bomb explosion with the force of an earthquake rocked the building, killing 6 people and injuring more than 1,000. (Xinhua/Wang Lei)

Family members of victims of the 1993 World Trade Center bombing attack participate in the 20th anniversary ceremony at Ground Zero in New York City on Feb. 26, 2013. The bomb explosion with the force of an earthquake rocked the building, killing 6 people and injuring more than 1,000. (Xinhua/Wang Lei)

Family members of victims of the 1993 World Trade Center bombing attack participate in the 20th anniversary ceremony at Ground Zero in New York City on Feb. 26, 2013. The bomb explosion with the force of an earthquake rocked the building, killing 6 people and injuring more than 1,000. (Xinhua/Wang Lei)

Family members of victims of the 1993 World Trade Center bombing attack participate in the 20th anniversary ceremony at Ground Zero in New York City on Feb. 26, 2013. The bomb explosion with the force of an earthquake rocked the building, killing 6 people and injuring more than 1,000. (Xinhua/Wang Lei)

Police officers participate in the 20th anniversary ceremony to commemorate victims of the 1993 World Trade Center bombing attack, at Ground Zero in New York City on Feb. 26, 2013. The bomb explosion with the force of an earthquake rocked the building, killing 6 people and injuring more than 1,000. (Xinhua/Wang Lei)

Port Authority Pipes and Drums band plays during the 20th anniversary ceremony to commemorate victims of the 1993 World Trade Center bombing attack, at Ground Zero in New York City on Feb. 26, 2013. The bomb explosion with the force of an earthquake rocked the building, killing 6 people and injuring more than 1,000. (Xinhua/Wang Lei)

Former New York City Mayor Dinkins (3rd R, Front) and New York City Mayor Bloomberg (2nd R) participate in the 20th anniversary ceremony to commemorate victims of the 1993 World Trade Center bombing attack, at Ground Zero in New York City on Feb. 26, 2013. The bomb explosion with the force of an earthquake rocked the building, killing 6 people and injuring more than 1,000. (Xinhua/Wang Lei)