Remembering 9/11 on the 16th anniversary of terrorist attacks

To mark the 16th anniversary of the terrorist attacks of Sept. 11, 2001, when al-Qaida militants hijacked airliners and flew them into the World Trade Center and the Pentagon, relatives of the nearly 3,000 people killed in the attacks gathered on Sept. 11, 2017, in New York, Pennsylvania, and outside Washington, D.C., to commemorate the loved ones they lost that day. (AP/Getty/Reuters)

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Remembering 9/11 on the 16th anniversary of terrorist attacks

A New York City Police Department (NYPD) officer breaks down while visiting the North pool during a commemoration ceremony for the victims of the September 11 terrorist attacks at the National September 11 Memorial, September 11, 2017 in New York City. (Photo: Drew Angerer/Getty Images)

Remembering 9/11 on the 16th anniversary of terrorist attacks

U.S. soldiers take part in a memorial ceremony to commemorate the16th anniversary of the 9/11 attacks, in Kabul, Afghanistan September 11, 2017. (Photo: Mohammad Ismail/Reuters)

Remembering 9/11 on the 16th anniversary of terrorist attacks

A flower is left at the North pool during a commemoration ceremony for the victims of the September 11 terrorist attacks at the National September 11 Memorial, September 11, 2017 in New York City. In New York City and throughout the United States, the country is marking the 16th anniversary of the September 11 terrorist attacks. (Photo: Drew Angerer/Getty Images)

Remembering 9/11 on the 16th anniversary of terrorist attacks

President Donald Trump stands for the national anthem during a ceremony to mark the anniversary of the Sept. 11 terrorist attacks, Sept. 11, 2017, at the Pentagon. (Photo: Evan Vucci/AP)

Remembering 9/11 on the 16th anniversary of terrorist attacks

A woman holds up a picture of a victim of the 9/11 terrorist attacks during a ceremony at ground zero in New York, Monday, Sept. 11, 2017. Holding photos and reading names of loved ones lost 16 years ago, 9/11 victims’ relatives marked the anniversary of the attacks at ground zero with a solemn and personal ceremony. (Photo: Seth Wenig/AP)

Remembering 9/11 on the 16th anniversary of terrorist attacks

A young boy carries an American flag as people gather at the National 911 Memorial and Museum during ceremonies marking the 16th anniversary of the September 11, 2001 attacks in New York, Sept. 11, 2017. (Photo: Brendan McDermid/Reuters)

Remembering 9/11 on the 16th anniversary of terrorist attacks

Flowers and mementos are seen left among names at the edge of the south reflecting pool at the National 9/11 Memorial and Museum during ceremonies marking the 16th anniversary of the September 11, 2001 attacks on the World Trade Center in New York, Sept. 11, 2017. (Photo: Brendan McDermid/Reuters)

Remembering 9/11 on the 16th anniversary of terrorist attacks

Flowers were left at the North pool memorial site before a commemoration ceremony for the victims of the Sept. 11 terrorist attacks at the National September 11 Memorial, Sept. 11, 2017, in New York City. In New York City and throughout the United States, the country is marking the 16th anniversary of the Sept. 11 terrorist attacks. (Photo: Drew Angerer/Getty Images)

Remembering 9/11 on the 16th anniversary of terrorist attacks

A makeshift memorial lines the fence on the 16th Anniversary of the attacks of September 11 at the World Trade Center 9/11 Memorial on Sept. 11, 2017, in New York. (Photo: Bryan R. Smith/AFP/Getty Images)

Remembering 9/11 on the 16th anniversary of terrorist attacks

President Trump and first lady Melania Trump stand for a moment of silence to mark the anniversary of the Sept. 11 terrorist attacks, on the South Lawn of the White House, Monday, Sept. 11, 2017, in Washington. (Photo: Evan Vucci/AP)

Remembering 9/11 on the 16th anniversary of terrorist attacks

A bouquet of roses is placed at a memorial wall outside Ladder 10 Engine 10 , Monday, Sept. 11, 2017, at the World Trade Center in New York. Thousands of 9/11 victims’ relatives, survivors, rescuers and others gathered Monday at the World Trade Center to remember the deadliest terror attack on American soil. During the attacks of Sept. 11, 2001, 343 firefighters were killed. (AP Photo/Mark Lennihan)

Remembering 9/11 on the 16th anniversary of terrorist attacks

New York City firefighters with Ladder 10 Engine 10 firehouse salute during a moment of silence, Monday, Sept. 11, 2017, at the World Trade Center in New York. Thousands of 9/11 victims’ relatives, survivors, rescuers and others gathered Monday at the World Trade Center to remember the deadliest terror attack on American soil. During the attacks of Sept. 11, 2001, 343 firefighters were killed. (AP Photo/Mark Lennihan)

Remembering 9/11 on the 16th anniversary of terrorist attacks

People pause at the edge of the South reflecting pool at the National September 11 Memorial and Museum during ceremonies marking the 16th anniversary of the Sept. 11, 2001, attacks in New York, Sept. 11, 2017. (Photo: Brendan McDermid/Reuters)

Remembering 9/11 on the 16th anniversary of terrorist attacks

Sen. Chuck Schumer, D-N.Y., speaks with New York Gov. Andrew Cuomo during a commemoration ceremony for the victims of the Sept. 11 terrorist attacks at the National September 11 Memorial, Sept, 11, 2017, in New York City. (Photo: Drew Angerer/Getty Images)

Remembering 9/11 on the 16th anniversary of terrorist attacks

Officers stand in silence at the National September 11 Memorial and Museum during ceremonies marking the 16th anniversary of the Sept. 11, 2001, attacks in New York, Sept. 11, 2017. (Photo: Brendan McDermid/Reuters)

Remembering 9/11 on the 16th anniversary of terrorist attacks

A woman wipes away tears as she holds a photograph of Alex Ciccone, a victim of the Sept. 11, 2001, attacks on the World Trade Center, at the National 9/11 Memorial and Museum during ceremonies marking the 16th anniversary of the attacks in New York, Sept. 11, 2017. (Photo: Brendan McDermid/Reuters)

Remembering 9/11 on the 16th anniversary of terrorist attacks

Police and Fire Department officers hold an American flag that flew over the World Trade Center at the National 911 Memorial and Museum during ceremonies marking the 16th anniversary of the Sept. 11, 2001, attacks in New York, Sept. 11, 2017. (Photo: Brendan McDermid/Reuters)

Remembering 9/11 on the 16th anniversary of terrorist attacks

New York City Mayor Bill de Blasio attends a commemoration ceremony for the victims of the Sept. 11 terrorist attacks at the National September 11 Memorial, Sept. 11, 2017 in New York City.(Photo: Drew Angerer/Getty Images)

Remembering 9/11 on the 16th anniversary of terrorist attacks

James Taormina, whose brother Dennis was killed in the 9/11 attacks, pauses at the memorial before a commemoration ceremony for the victims of the attacks at the National September 11 Memorial, Sept. 11, 2017, in New York City. (Photo: Drew Angerer/Getty Images)

Remembering 9/11 on the 16th anniversary of terrorist attacks

President Trump and first lady Melania Trump attend a ceremony at the Pentagon’s 9/11 Memorial in Washington, D.C., on Sept. 11, 2017, on the 16th anniversary of 9/11. (Photo: Brendan Smialowski/AFP/Getty Images)

Remembering 9/11 on the 16th anniversary of terrorist attacks

An American flag is left at the North pool memorial site before a commemoration ceremony for the victims of the September 11 terrorist attacks at the National September 11 Memorial, September 11, 2017 in New York City. In New York City and throughout the United States, the country is marking the 16th anniversary of the September 11 terrorist attacks. (Photo: Drew Angerer/Getty Images)

Remembering 9/11 on the 16th anniversary of terrorist attacks

Two women visit the September 11th Memorial at the Pentagon on the 16th anniversary of the September 11th attacks, Monday, Sept. 11, 2017. (Photo: Jacquelyn Martin/AP)

Remembering 9/11 on the 16th anniversary of terrorist attacks

President Trump and first lady Melania Trump stand for a moment of silence to mark the anniversary of the Sept. 11 terrorist attacks, on the South Lawn of the White House, Monday, Sept. 11, 2017, in Washington. (Photo: Evan Vucci/AP)

Remembering 9/11 on the 16th anniversary of terrorist attacks

The National September 11 Memorial and Museum are set for a memorial service, Monday, Sept. 11, 2017, in New York. Thousands of 9/11 victims’ relatives, survivors, rescuers and others are expected to gather Monday at the World Trade Center to remember the deadliest terror attack on American soil. Nearly 3,000 people died when hijacked planes slammed into the trade center, the Pentagon and a field near Shanksville, Pa., on Sept. 11, 2001. (Photo: Mark Lennihan/AP)

Remembering 9/11 on the 16th anniversary of terrorist attacks

A U.S. flag is unfurled at the Pentagon on the 16th anniversary of the Sept. 11 attacks Monday, Sept. 11, 2017. (Photo: Jacquelyn Martin/AP)

Remembering 9/11 on the 16th anniversary of terrorist attacks

Visitors unfurl a commemorative flag at the Flight 93 National Memorial on the 16th Anniversary ceremony of the Sept. 11 terrorist attacks, Sept. 10, 2017, in Shanksville, Pa. United Airlines Flight 93 crashed into a field outside Shanksville with 40 passengers and four hijackers aboard on Sept. 11, 2001. (Photo: Jeff Swensen/Getty Images)