NEW YORK — The workers in Nancy Cassidy's office on the 80th floor of Two World Trade Center had a head start on their escape: They fled after a jet smashed the neighboring tower.

"The fireballs flew past my window, and I hung up the phone," said Cassidy, 42, personnel manager for the Mizuho Capital Markets trading company. "I went out of my private office, and my boss was screaming, 'Get out, get out!' "

Fifty people on her side of the office had cleared out by the time she started down.

"All of a sudden you heard, 'Shhh,' " she said. "Everyone was quiet. That's when they made that announcement: 'Building 1 is in a state of emergency. Building 2 is secure. You're fine, you can return to your workstations.' "

It was an awful dilemma for the building's fire-safety staff, experts said. If workers left the as-yet-unharmed tower, they could be hit by falling glass, concrete or aircraft parts. And high-rises are considered generally safe during fires, with their sprinkler systems and fire-resistant walls. On the other hand, the twin towers were under an unprecedented attack.

The workers in Two World Trade Center faced the same choice.

"It was thousands of people, and you could hear a pin drop in that stairway," Cassidy said. "It was incredible. People were like, 'We're done, we're not going back up. We're leaving anyway.' " When a second hijacked jet struck the second tower 18 minutes after the first, Cassidy was already down to the 44th floor.

"When the plane hit our tower, you heard a boom, and the building shook, the stairwell shook," she said. "It was so scary."

Tiffany Keeling, 32, of Albuquerque was among 350 Morgan Stanley financial advisers in training on the 61st floor of the second tower. She said her group also started to head downstairs immediately after the first jet struck.

She, too, heard the announcement.

"An announcement came over the speaker system that we were not in imminent danger and that we should return to our offices," Keeling said. "I continued down the stairs, and that's when the second plane hit. I could smell the jet fuel."

Dan Baumbach, 24, a software engineer from Long Island, was stunned to find that building officials in One World Trade Center, the first tower to be struck, were telling workers not to evacuate, even after it was hit by a jet.

"You can try it, but it's at your own risk," he quoted one official as telling a group of 100 people on the 75th floor. Many chose to follow that advice; Baumbach continued his descent from the 80th floor and survived.

"The reason we got out was because we didn't listen," he said.

Officials of the Port Authority of New York and New Jersey, which runs the trade center, declined to discuss the evacuation. "I have no comment on anything relating to that incident," said Ernesto Butcher, the chief operating officer.

A senior law-enforcement official said there was conflicting information, adding authorities had no way of knowing for sure what had happened.

Robert Solomon of the National Fire Protection Association said the safety staff was in a difficult position. Once workers in the second tower left, he said, they would be exposed to falling debris.

"It's a terrible dilemma," he said. "If the second plane doesn't come, it makes sense to stay where you are now."

But Cassidy, who had the job of giving out safety kits to new employees, said she had to wonder about those who heeded the announcement. "It could be that, because of that announcement, some people from my company went back upstairs and now may be gone," she said.

On the 87th floor of Tower Two, Louis Giaccardo knew immediately that he wanted to get out after he saw an orange fireball sweep across his window after the plane slammed into the first tower.

"What was that?" exclaimed a colleague at Corporation Service, a firm that processes incorporation papers for state and county governments. Moments later, Giaccardo, 35, a Haddon Township, Pa., father of two, smelled jet fuel and understood he and the 20 or so people in his office were in trouble.

"Get off the floor!" he screamed. People raced for the elevators. They rode for nine floors to switch to express elevators.

But too many others were there, and the group began to panic, so Giaccardo headed down the stairs.

Sirens were blaring, and as he reached the 40th floor, he heard an astonishing announcement from the loudspeakers: "Remain calm and stay in the building."

When he reached the 38th floor, he heard a thunderous boom — the second plane hitting Giaccardo's building well above where he stood.

A rumble rippled down the stairwell as everyone froze in silence. "I thought, 'I am going to die in this stairwell,' " Giaccardo said. After he escaped, Giaccardo looked back from about two blocks away. There was a gaping hole on the 87th floor — his floor.

Aaron Goldsmid, a 21-year-old Columbia University student and part-time computer worker at Lehman Bros., was on the 40th floor of the first tower when the first plane hit.

"Everything shook," he said. "The lights dimmed. I saw a hundred people, some of them panicking."

As he hurried downstairs, firefighters streamed past him. "One of the hardest things was watching rescue workers go into the center," he said.

In a short while, he'd learn that many of them never got out alive.

Information from Knight Ridder Newspapers is included in this report.