At least 45 people are now known to have died after a fourth hijacked plane crashed in Pennsylvania yesterday afternoon.
The plane, believed to be a Boeing 767, crashed about eight miles east of Jennerstown in countryside.
It is believed that Flight 93 was apparently intended for Camp David, the presidential retreat in the mountains of Maryland.
State Police Major Lyle Szupinka says there is no reason to believe there were any survivors of the crash.
James Moran, a Virginia Democrat, said Camp David was the plane's target after a Marine Corps briefing in Washington
The crash site was 85 miles northwest of Camp David.
"There's a crater gorged in the earth, the plane is pretty much disintegrated. There's nothing left but scorched trees," said Mark Stahl of Somerset, who went to the scene.
United said Flight 93 left Newark at 8:01 am (1301 BST) with 38 passengers, two pilots and five flight attendants.