The last piece of the World Trade Center still standing aboveground — a battered but recognizable staircase used by hundreds to flee the inferno of 9/11 — is one of the most endangered historical places in America, the National Trust for Historic Preservation said yesterday.

That is because it stands in the way of an office tower designed by Norman Foster of London and planned by Larry A. Silverstein, president of Silverstein Properties.

"Silverstein Properties has not made a commitment to preserve the staircase and we're urging them to do so," said Richard Moe, the president of the trust, a nonprofit group that has no regulatory power but does have influence in rallying attention to historically significant places.