9/11 memorial

A woman (C) walks amongst US national flags erected by students and staff from Pepperdine University to honor the victims of the September 11, 2001 attacks in Malibu, California on September 9, 2014. The students placed some 3,000 flags in the ground in tribute to the nearly 3,000 victims lost in the attacks almost 13 years ago. AFP PHOTO / Mark RALSTONMARK RALSTON/AFP/Getty Images

Mark Ralston/AFP/Getty Images

A woman walks among some 3,000 US national flags erected by students and staff from Pepperdine University Malibu, to honor the victims of the September 11 2001 attacks

The Tribute in Light illuminates the sky in New York on the 12th anniversary of the September 11, 2001 attacks. The tribute, an art installation of the Municipal Art Society, consists of 88 searchlights placed next to the site of the World Trade Center creating two vertical columns of light.

Timothy Clary/AFP/Getty Images

The Tribute in Light illuminates the sky in New York on the 12th anniversary of the September 11 2001 attacks. The tribute, an art installation of the Municipal Art Society, consists of 88 searchlights placed next to the site of the World Trade Center, creating two vertical columns of light

A man walks through the 9/11 Empty Sky memorial at sunrise across from New York's Lower Manhattan and One World Trade Center in Liberty State Park in Jersey City, New Jersey.

Gary Hershorn/Reuters

A man walks through the 9/11 Empty Sky memorial at sunrise across from New York’s Lower Manhattan and One World Trade Center in Liberty State Park in Jersey City, New Jersey.

A girl plants some of the 3000 flags placed in memory of the lives lost in the September 11, 2001 attacks, at a park in Winnetka, Illinois

Reuters/Jim Young

A girl plants some of the 3,000 flags placed in memory of the lives lost in the September 11 2001 attacks, at a park in Winnetka, Illinois, US