A visitor pays tribute to the passengers and crew of Flight 93 at the Flight 93 National Memorial in Shanksville, Pa, Sunday, Sept. 11, 2016. (AP Photo/Jared Wickerham)
Mourners hold photos of their loved ones during the 15th anniversary of the attacks of the World Trade Center at the National September 11 Memorial, Sunday, Sept. 11, 2016. (AP Photo/Mary Altaffer)
Democratic presidential candidate Hillary Clinton arrives to attend a ceremony at the National September 11 Memorial, in New York, Sunday, Sept. 11, 2016. (AP Photo/Andrew Harnik)
A mourner holds a photo of her loved one during the 15th anniversary of the attacks of the World Trade Center at the National September 11 Memorial, Sunday, Sept. 11, 2016. (AP Photo/Mary Altaffer)
Mourners hold a photo of their loved one during the 15th anniversary of the attacks of the World Trade Center at the National September 11 Memorial, Sunday, Sept. 11, 2016. (AP Photo/Mary Altaffer)
A mourner holds a photo of their loved one during the 15th anniversary of the attacks of the World Trade Center at the National September 11 Memorial, Sunday, Sept. 11, 2016. (AP Photo/Mary Altaffer)
Republican presidential candidate Donald Trump, left, speaks with New Jersey Gov. Chris Christie, right, as he attends a ceremony at the Sept. 11 memorial. (AP Photo/Andrew Harnik)
Republican presidential candidate Donald Trump, center, attends a ceremony at the Sept. 11 memorial, in New York, Sunday, Sept. 11, 2016. (AP Photo/Andrew Harnik)
President Barak Obama bows his head for a moment of silence before delivering remarks at the Sept. 11 memorial observance ceremony at the Pentagon, Sunday, Sept. 11, 2016. (AP Photo/Cliff Owen)
A couple hugs while sitting in the Pentagon Memorial prior to a Sept. 11 observance ceremony at the Pentagon, Sunday, Sept. 11, 2016. (AP Photo/Cliff Owen)
The U.S. marked the 15th anniversary of 9/11 on Sunday, with victims' relatives reading their names and reflecting on a loss that still felt as immediate to them as it was indelible for the nation.