Flight 93 National Memorial

Brent Everitt, public information officer from The Great Smoky National Park, looks at the Flight 93 National Memorial Visitor Center on Sept. 9, 2015. The 40 ft high concrete walls that represent the 40 passengers and crew while The Entry Portal, a black granite walkway, follows Flight 93's final path before it crashed on September 11, 2015.

Construction continues on the Flight 93 National Memorial in Shanksville on Thursday, June 16, 2011.

Former Presidents George W. Bush and Bill Clinton stand near the wall of names with Vice President Joe Biden and Secretary of the Interior Ken Salazar at the United Flight 93 National Memorial in Shanksville on Saturday, September 10, 2011.

Alice Hoagland, the mother of Mark Bingham, stands by her son's name on the wall of names at the United Flight 93 National Memorial in Shanksville on Saturday, September 10, 2011.

President Obama and First Lady Michelle arrive at the Flight 93 National Memorial near Shanksville on September 11, 2011, where they laid a wreath at the Wall on Names.

Visitors at the Flight 93 National Memorial near Shanksville take photographs of the crash site through the Ceremonial Gate on Monday, September 10, 2012.

Donna Glessner, Vice Chair of the Flight 93 Federal Advisory Commission, shows Former Ambassador Dan Rooney the site of the Visitor Center Complex at the Flight 93 National Memorial during a Groundbreaking Ceremony.

Keith Newlin, deputy superintendent of the National Park Service, holds a digital image before a media tour of the Flight 93 National Memorial Visitor Center Complex, which includes the visitor center, flight path walkway and the learning center, all in different stages of construction, on Wednesday, Sept. 10, 2014 near Shanksville.

Emily Schenkel, god child of Flight 93 flight attendant Lorraine Bay looks at a display featuring photographs of passengers and crew members at the Flight 93 National Memorial Visitor Center on Sept. 9, 2015.

Construction vehicles work in the background at the temporary Flight 93 memorial near Shanksville, Pa., on May 2, 2011.

Carole Hoffman of Allentown becomes emotional at the dedication of the Flight 93 National Memorial Visitor Center on Thursday, Sept. 10, 2015 near Shanksville.

A woman stands against the wall of names at the Flight 93 National Memorial in Stony Creek Township on Saturday, Sept. 10, 2016. An evening luminaria program was held placing candles in lanterns at every name on the wall.

A park ranger stands at the observation point of the Flight 93 National Memorial in Stonycreek Township on Sunday, Sept. 11, 2016.

Luis Daniel Rodriguez, Jr., 11, of Newark, looks out over the Flight 93 National Memorial fields in Shanksville with his fellow singers from the Newark Boys Chorus on the 15th anniversary of 9/11 on Sunday, Sept. 11, 2016. The choir sang at the service of remembrance for the 40 lives lost in the crash at the site.

The Lacey family of Jessup, Md. gather for 'A Service of Remembrance' at the Flight 93 National Memorial in Shanksville for the 15th anniversary of 9/11 on Sunday, Sept. 11, 2016. From left to right are Emma, 5, Keith, 38, Ruth, 12, Sarah, 35, Katherine, 1, and Gavin, 7.

Vice President Mike Pence, and Second Lady Karen Pence, make their way along the wall of names, to place a wreath at the wall, during the 16th annual 9/11 memorial service, at the Flight 93 National Memorial, in Stoney CreekTownship, on Monday, Sept. 11, 2017.

Construction on the Tower of Voices continues at the Flight 93 National Memorial, in Shanksville, on Monday, June 11, 2018.

A couple looks toward the Wall Of Names from the Flight Path Overlook at the Flight 93 National Memorial on Sunday, April 24, 2016, in Stony Creek.

Flight 93 family members Campbell Peterson (left), Peyton Peterson (second from left), Sarah Wainio, and Justin Nacke join a ground breaking ceremony for the Flight 93 National Memorial near Shanksville on November 7, 2009.

Volunteers Daniel Zwick, of the Shanksville area, and Mary Alice Mankamyer, of Shanksville, remove items from a memorial fence at the Flight 93 temporary memorial near Shanksville, Pa. on July 22, 2008. A property dispute between the National Park Service and the current landowner is forcing officials to move the temporary memorial to a site across the road from its current location.

Gordon W. Felt, the brother of Flight 93 passenger Edward P. Felt, speaks with Senator John McCain prior to a memorial service for the passengers and crew of Flight 93 at the temporary memorial near Shanksville on September 11, 2008.

United States Secretary of the Interior Ken Salazar is introduced during a ground breaking ceremony for the Flight 93 National Memorial near Shanksville on November 7, 2009.

Secretary of Defense Donald Rumsfeld looks at the Angels of Freedom during his visit to the Flight 93 Temporary Memorial in Somerset County on March 27, 2006.

As United State Senator Arlen Specter, left, signs a National Park Services document Joanne M.Hanley, superintendent of the Flight 93 National Memorial, center and Pennsylvania Governor Edward Rendell, right, share a laugh after he too signed the document prior on Friday afternoon in Somerset County Feb. 20, 2009. Senators Arlen Specter and Bob Casey, who didn't show up for the event, and Governor Edward G. Rendell joined with the Dan Wenk, the Acting Director of the National Park Service, as well as Flight 93 National Memorial officials announced forward plans regarding the Flight 93 National Memorial. S.C. Spangler/Tribune-Review scs flight93 21 01 (story for mary pickles )

Hand written notes and memorabilia are left at the temporary Flight 93 national memorial on May 10, 2006.

General Colin Powell (left), Secretary of the Interior Ken Salazar (middle), and Families of Flight 93 President Gordon Felt bow their heads during a memorial service marking the eighth anniversary of the 9/11 attacks near Shanksville on September 11, 2009.

Shovels lean against a table before a ground breaking ceremony for the Flight 93 National Memorial near Shanksville on November 7, 2009.

A cross stands as a memorial at the crash site of United Flight 93 in Shanksville, Somerset County, PA near the site where 40 people perished on September 11, 2001. The site has been a sacred place for many who come from across the country to stand in the rural Pennsylvania field often leaving mementos such as notes, flags, or coins just to leave sometime in tribute to the heros of Flight 93.

Construction continues around the Flight 93 crash site near Shanksville on July 6, 2010.

Paul Murdoch, architect of the Flight 93 National Memorial, discusses the design and progress of the construction at the temporary memorial area near the Western Overlook in Shanksville, PA on August 12, 2010.

First lady Michelle Obama claps as former first lady Laura Bush makes her way to the podium during the ninth anniversary commemoration of the September 11, 2001 terrorist attacks at the Flight 93 National Memorial near Shanksville, Somerset County.

Construction continues on a permanent memorial at the Flight 93 crash site near Shanksville on April 14, 2011.

Construction continues on the permanent memorial at the crash site of United Flight 93 in Shanksville, PA on April 21, 2011.

Construction continues on the permanent memorial at the crash site of United Flight 93 in Shanksville, PA on April 21, 2011.

Construction continues on the permanent memorial at the crash site of United Flight 93 in Shanksville, PA on April 21, 2011.

Jeff Ray, of Berlin, Pa., kisses his wife Barbara while visiting the temporary Flight 93 memorial near Shanksville, Pa., on May 2, 2011.