US Vice President Joe Biden and former Presidents Bill Clinton and George W. Bush are helping dedicate a memorial Saturday near Shanksville, Pennsylvania, where United Flight 93 crashed on Sept. 11.
Forty passengers and crew died on board the aircraft, which plummeted to earth during an onboard struggle to thwart hijackers who had seized the plane.
A temporary memorial has stood for a decade at the site in southwestern Pennsylvania, and nearly 1.5 million people have visited.
The two-hour ceremony, to begin at 12:30 p.m./ 1630 GMT will include remarks, musical tributes and the laying of wreaths.
Canadian singer-songwriter Sarah McLachlan will perform.
Other dignitaries scheduled to attend include Republican Speaker of the House of Representatives, John Boehner, Vice President Biden's wife, Jill Biden, former first lady Laura Bush, Secretary of the Interior Ken Salazar and Pennsylvania Governor Tom Corbett, organizers said.
President Barack Obama is scheduled to visit the site on Sunday, the 10th anniversary of the attacks.
Flight 93 crashed at 10:03 a.m. on Sept. 11, 2001.
Organizers say $10 million still needs to be raised to complete construction of the Flight 93 National Memorial.
About $52 million has already been raised in public and private funds.
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