Chief Palmer began his assignment in the north tower after the first plane struck, helping to organize the operations there. Soon after the second plane hit the south tower at 9:02 a.m., Chief Palmer moved into that building with Chief Burns.

Although most elevators were knocked out of service, Chief Palmer found one that was working and took it to the 41st floor. At that point, he was halfway to the impact zone, which ran from the 78th to the 84th floors.

As he began climbing, he crossed paths with a handful of injured people who had been in the 78th floor Sky Lobby, where scores of office workers had been waiting for express elevators when the second plane hit. The tip of its left wing grazed the lobby, instantly killing most of a group variously estimated between 50 and 200 people. Only a dozen ultimately escaped from the building. Among them was Judy Wein.

''We saw the firefighters coming up, and they would ask us, what floor did you come from?'' Ms. Wein recalled in an interview. ''We told them, 78, and there's lots of people badly hurt up there. Then they would get on their walkie-talkies and report back in.''

Ed Nicholls, whose arm was nearly severed by the blast across the 78th floor, recalled in an interview that he saw a firefighter somewhere around the 50th floor who had advice on how to get out. ''We encountered a fireman who told us to go to the 41st floor,'' he said.

While it is impossible to say if Chief Palmer was the firefighter whom Mr. Nicholls saw, the chief did send radio messages with the information that he collected from civilians trying to escape the building.

As Ling Young, another survivor of the 78th floor, made her way down, she passed two fire marshals, Mr. Bucca and James Devery. They had climbed the stairs from the lobby because they did not know about the elevator that ran to the 41st floor. ''Ronnie was ahead of me, like a flight, at all times -- he was just in better shape,'' Mr. Devery said in an interview. ''And then on the 51st floor there was a woman standing there on the stairwell landing and she had her arms out and her eyes were closed. And she was bleeding from the side.'' That was Mrs. Young, and she seemed ready to faint, he recalled, so he decided to escort her out.