In pictures: The U.S. commemorates the 25th anniversary of 9/11

On Sept. 11, 2001, hijackers flew planes into the World Trade Center and other monuments, killing thousands. 25 years later, photos capture those marking 9/11.On Sept. 11, 2001, hijackers flew planes into the World Trade Center and other monuments, killing thousands. 25 years later, photos capture those marking 9/11.