Visitors walk around the Pepperdine University's annual display of flags honoring the victims of the 9/11 terrorist attacks, Monday, Sept. 10, 2018, in Malibu, Calif.
FILE - In this Sept. 11, 2001 photo, the twin towers of the World Trade Center burn behind the Empire State Building in New York after terrorists crashed two planes into the towers causing both to collapse.
Visitors to the Flight 93 National Memorial in Shanksville, Pa., participate in a sunset memorial service on Monday, Sept. 10, 2018, as the nation marks the 17th anniversary of the Sept. 11, 2001 attacks.
1/15President Donald Trump and first lady Melania Trump participate in the September 11th Flight 93 memorial service in Shanksville, Pennsylvania, Sept. 11, 2018.
2/15A woman leaves flowers at the North Pool during a ceremony marking the 17th anniversary of the terrorist attacks on the United States, Sept. 11, 2018, in New York.
3/15Vice President Mike Pence, center, speaks during the September 11th Pentagon Memorial Observance at the Pentagon, Sept. 11, 2018.
4/15A guest wipes tears at the south reflecting pool of the National 9/11 Memorial and Museum during ceremonies marking the 17th anniversary of the September 11, 2001 attacks on the World Trade Center in New York, Sept. 11, 2018.
5/15A ribbon and roses are placed on one of the victims' name at the north reflecting pool of the National 9/11 Memorial in New York, Sept. 11, 2018.
6/15People hold up photo memorials during a ceremony marking the 17th anniversary of the terrorist attacks on the United States, Sept. 11, 2018, in New York.
7/15La-Shawn Clark, left, and Mark Cannizzaro read names of victims of the Sept. 11 attacks, Sept. 11, 2018, in New York. Clark lost her husband, Benjamin Clark, and Cannizzaro lost his cousin, Brian Cannizzaro.
8/15Bill Anders, left, and Sharon Custer ring bells as part of the Name Presentation and Ringing of Bells Remembrance during the September 11th Flight 93 Memorial Service in Shanksville, Pennsylvania, Sept. 11, 2018.
9/15A U.S. flag is unfurled at sunrise on Sept. 11, 2018, at the Pentagon.
10/15New York City firefighters salute in front of a memorial outside a firehouse adjacent to One World Trade Center and the 9/11 Memorial site in New York, Sept. 11, 2018.
11/15Family members of Flight 93 observe the September 11th Flight 93 Memorial in Shanksville, Pennsylvania, Sept. 11, 2018.
12/15Guests mark the 17th anniversary of the 9/11 attacks on the World Trade Center, at the National 9/11 Memorial and Museum in New York, Sept. 11, 2018.
13/15A woman carries a photo of Wilder Gomez who was a bartender at Windows on the World on the 103rd floor of the World Trade Center, during the 17th anniversary of the terrorist attacks on the United States, Sept. 11, 2018, in New York.
14/15Visitors to the Flight 93 National Memorial in Shanksville, Pa., participate in a sunset memorial service, Sept. 10, 2018, as the nation marks the 17th anniversary of the Sept. 11, 2001 attacks.
15/15Bianca Burton, center left, and Erin Schultheis walk around the Pepperdine University's annual display of flags honoring the victims of the 9/11 terrorist attacks, Sept. 10, 2018, in Malibu, California.
