President Barack Obama and first lady Michelle Obama are accompanied by Air 

President Barack Obama and first lady Michelle Obama are accompanied by Air Force Col. William Knight and his wife Susan as they walk from the Marine One helicopter  to board Air Force One at Andrews Air Force Base, Md., Wednesday, April 24, 2013, as they travel to Dallas to attend the George W. Bush presidential library dedication and the memorial for victims of the fertilizer plant explosion in West, Texas. (AP Photo/Charles Dharapak)
Associated Press
President Barack Obama and first lady Michelle Obama are accompanied by Air Force Col. William Knight and his wife Susan as they walk from the Marine One helicopter to board Air Force One at Andrews Air Force Base, Md., Wednesday, April 24, 2013, as they travel to Dallas to attend the George W. Bush presidential library dedication and the memorial for victims of the fertilizer plant explosion in West, Texas. (AP Photo/Charles Dharapak)
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