TWO YEARS LATER: THE 91ST FLOOR; The Line Between Life and Death, Still Indelible

Everyone above them died.

Even two years later, that unavoidable fact lingers, inspiring guilt, instilling resolve and shaping decisions big and small.

Ever since 9/11, Claire McIntyre and others in her small company, American Bureau of Shipping, have had to grapple with this stark truth: They represent the line between life and death in the north tower of the World Trade Center.

A.B.S. was on the 91st floor. The 1,344 people who were on the 19 floors above theirs perished, while all 11 A.B.S. employees in the office that day and nearly everyone below them descended to safety.

The realization, grasped only gradually as details of the catastrophe emerged, proved paralyzing, energizing and confounding all at once. Merline Mayers, a business analysis coordinator, fought back paranoia; Raymond Ng, her office mate, chose to pretend nothing happened at all; Ms. McIntyre, the company's office manager, wrestled with whether she should be doing more with her life.

''I know I should be doing something,'' Ms. McIntyre said. ''I don't know what.''

On Sept. 11, 2001, Ms. McIntyre, 49, had been in the office a half-hour when she heard the roar of a passenger jet. Out the window, she glimpsed a wing and tail just as it barreled into the building just above her. The plane had come in at an angle, tearing across floors 94 to 98 and igniting a ferocious fire. Ms. McIntyre screamed and ran into the hallway, yelling for everyone to get out.

George Sleigh, a 65-year-old British-born engineer, occupied the easternmost office, closest to the point of impact. The ceiling above him crumbled, but he was able to crawl out.

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