PHOTO: EAST NEWS/AFP PHOTO Investigative personnel search the crash site of United Airlines Flight 93 looking for debris and evidence, including the plane's flight recorder, 12 September 2001 in Shanksville, Pennsylvania. The hijacked plane crashed killing all 45 people on board.

People attending the dedication stand around the 93-foot tall Tower of Voices on Sunday, Sept. 9, 2018 at the Flight 93 National Memorial in Shanksville, Pa. The tower contains 40 wind chimes representing the 40 people that perished in the crash of Flight 93 in the terrorist attacks of Sept. 11, 2001.(AP Photo/Keith Srakocic, Pool), APTOPIX

US President Barack Obama and First Lady Michelle Obama walk hand in hand on September 11, 2011 as they visit the crash site after attending in a commemoration ceremony at the Flight 93 National Memorial on the 10th anniversary of the 9/11 attacks in Shanksville, Pennsylvania. AFP PHOTO/Mandel NGAN Attn Editors : These file pictures are part of a "Best of" serie of the US President Barack Obama two terms in office. / AFP PHOTO / MANDEL NGAN

Edward Noon, 22, of the Pennsylvania National Guard, pays tribute to the passengers and crew of Flight 93, while candles are lit along the Wall of Names at the Flight 93 National Memorial in Shanksville, Pa, Saturday, Sept. 10, 2016, as the nation marks the 15th anniversary of the Sept. 11 attacks. (AP Photo/Jared Wickerham), APTOPIX

Members of the media get a preview of the the Flight 93 National Memorial visitors center complex in Shanksville, Pa., Wednesday, Sept. 9, 2015. The visitors center will be formally dedicated and open to the public on Sept. 10, 2015. (AP Photo/Gene J. Puskar), APTOPIX

A makeshift memorial to victims of Flight 93, which crashed on September 11, 2001, in Shanksville, PA, a few miles off of the Lincoln Highway. The Lincoln Highway, the first cross-country road in the United States, was founded in 1913 and celebrates its 100th anniversary this year. Roughly 3300 miles long, the road passes through a wide variety of landscapes and more than 700 towns, cities, and communities as it winds its way from Times Square to its western terminus in Lincoln Park, San Francisco.

Candles in memory of the passengers and crew of Flight 93, are lit along the Wall of Names at the Flight 93 National Memorial in Shanksville, Pa., Saturday, Sept. 10, 2016, as the nation marks the 15th anniversary of the Sept. 11 attacks. (AP Photo/Jared Wickerham), APTOPIX