Slideshow

Sept. 11 15th Anniversary

01 / 15

15th Anniversary of 9/11

American flags are placed on the south pool of the National September 11 Memorial, Saturday, Sept. 10, 2016, in New York. Sunday marks the 15th anniversary of the attacks on the World Trade Center. (AP Photo/Mary Altaffer)

02 / 15

15th Anniversary of 9/11

An FDNY firefighter stands as part of an honor guard at the FDNY Memorial Wall near the World Trade Center on Sunday, Sept. 11, 2016, on the 15th anniversary of the attacks on the World Trade Center in New York. (AP Photo/Craig Ruttle)

03 / 15

15th Anniversary of 9/11

FDNY firefighters, perform the changing of the guard at the FDNY Memorial Wall near the World Trade Center on Sunday, Sept. 11, 2016, on the 15th anniversary of the attacks on the World Trade Center in New York. (AP Photo/Craig Ruttle)

04 / 15

15th Anniversary of 9/11

An FDNY firefighter walks among flags at the FDNY Memorial Wall near the World Trade Center Sunday, Sept. 11, 2016, on the 15th anniversary of the attacks on the World Trade Center in New York. (AP Photo/Craig Ruttle)

05 / 15

15th Anniversary of 9/11

Mourners observe a moment of silence during the 15th anniversary of the Sept. 11 attacks at the National September 11 Memorial, Sunday, Sept. 11, 2016, in New York. (AP Photo/Mary Altaffer)

06 / 15

15th Anniversary of 9/11

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, in New York. (AP Photo/Mary Altaffer)

07 / 15

15th Anniversary of 9/11

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, in New York. (AP Photo/Mary Altaffer)

08 / 15

15th Anniversary of 9/11

A visitor to the National September 11 Memorial arranges flowers on the north pool, Saturday, Sept. 10, 2016, in New York. Sunday marks the 15th anniversary of the attacks on the World Trade Center. (AP Photo/Mary Altaffer)

09 / 15

15th Anniversary of 9/11

American flags and a ribbon decorate a name carved into the north pool of the National September 11 Memorial on the 15th anniversary of the attacks of the World Trade Center, Sunday, Sept. 11, 2016 in New York . (AP Photo/Mary Altaffer)

10 / 15

15th Anniversary of 9/11

A firefighter touches the names of the firefighters who died in the Sept. 11 attacks carved in the south pool during 15th anniversary ceremony of the attacks on the World Trade Center at the Sept. 11 memorial, Sunday, Sept. 11, 2016, in New York. (AP Photo/Mary Altaffer)

11 / 15

15th Anniversary of 9/11

A photograph and roses decorate a name carved into the north pool of the National September 11 Memorial on the 15th anniversary of the attacks of the World Trade Center, Sunday, Sept. 11, 2016 in New York. (AP Photo/Mary Altaffer)

12 / 15

15th Anniversary of 9/11

A blue and white ribbon decorates a name carved in the south pool of the National September 11 Memorial on the 15th anniversary of the attacks of the World Trade Center, Sunday, Sept. 11, 2016 in New York. (AP Photo/Mary Altaffer)

13 / 15

15th Anniversary of 9/11

A tour guide, right foreground holding yellow flower, talks to visitors in front of Ladder Co. 10 Engine Co. 10 during a tour of the National September 11 Memorial, Saturday, Sept. 10, 2016, in New York. Sunday marks the 15th anniversary of the attacks on the World Trade Center. (AP Photo/Mary Altaffer)

14 / 15

15th Anniversary of 9/11

The Tribute in Light, viewed from the Brooklyn borough of New York, rises above the lower Manhattan skyline, Sunday, Sept. 11, 2016, in New York, the fifteenth anniversary of the terrorist attacks of Sept. 11, 2001 on the United States. (AP Photo/Craig Ruttle)

15 / 15

15th Anniversary of 9/11

Blue beams of the Tribute of Light pierce the air over near the National September 11 Memorial and Museum in the Manhattan borough, seen from the Brooklyn borough in New York on Sunday, Sept. 11, 2016. Sunday marked the 15th anniversary of the September 11 attacks on the United States. (AP Photo/Hiro Komae)