U.S. President Donald Trump will speak during a ceremony held at the Flight 93 National Memorial here, marking the 17th anniversary of the Sept. 11 attacks.
The memorial is located at the site of the crash of United Airlines Flight 93, which was hijacked 17 years ago, in Somerset County, state of Pennsylvania.
The event starts at 9:45 a.m.(1445 GMT), a time chosen to commemorate the time when the passengers and crew acted on a plan to thwart the terrorists aboard the flight.
The open-air service on Memorial Plaza will also include the reading of the names of the 40 passengers and crew members of Flight 93, ringing of the Bells of Remembrance and a wreath-laying at the Wall of Names.
The White House said first lady Melania Trump will accompany the president.
With Trump attending Tuesday's observance of the Flight 93 crash, the event could bring a crowd of as many as 5,000 people, National Park Service officials said Monday.
Nearly 3,000 people were killed on Sept. 11, 2001, when hijackers flew commercial airplanes into the World Trade Center in New York City, the Pentagon in Washington, D.C., and the field in Somerset County.
Observances will also be held at the World Trade Center and the Pentagon on Tuesday.
Trump observed the somber anniversary for the first time as president last year at the White House with a moment of silence at the exact time the first plane hit the Twin Towers.
He also participated in last year's 9/11 observance at the Pentagon.
Trump will be the second sitting president to visit the Flight 93 Memorial on the anniversary date. Former U.S. President George W. Bush visited the site in 2006.
The memorial, named the Tower of the Voices, features 40 wind chimes, whose sound honors each of the passengers and crew members of Flight 93.