When the first aircraft struck the World Trade Center on Tuesday, employees of the biggest tenant in the complex knew exactly what to do. Now, all but a handful of Morgan Stanley's 3,700 employees are alive to talk about it.

In 1993, when terrorists bombed the World Trade Center, Dean Witter, a brokerage that is now part of Morgan Stanley, was among those affected. Employees have been training ever since in case the madness returned.

The workers - mostly involved in processing trades for retail investors or handling administrative matters - were organised floor by floor to evacuate. Drills were conducted and a back-up facility was maintained nearby in lower Manhattan.

"If you went though 1993, it was very, very scary," said Philip Purcell (above left), Morgan Stanley's chairman and chief executive officer. "The lessons were very, very clear."

After the first aircraft crashed into the north tower, Morgan Stanley's employees in the south tower headed out - and never turned back. By the time the second aircraft struck just above Morgan Stanley's 22 floors in the south tower, the evacuation was well under way.

As a result of their preparation - and no small amount of luck - all but a handful of Morgan Stanley's workers made it to safety.

But by the time they reached the streets below, the lessons of 1993 no longer applied. This time, there was no turning back. Morgan Stanley had to reorganise itself on the run and even now no one can be sure how long that process will take.

Some workers were so traumatised they went home and avoided contact. Days went by, and no one in headquarters heard from them.

"Everybody is in shock," Mr Purcell said in an interview more than two days after the attack. "The people who got out are hurt. They watched people jump out of the buildings."

To gather its employees, Morgan Stanley put its global communications network to internal use. A call centre in Phoenix, Arizona, that normally serves its Discover credit card unit was charged with reaching out to workers.

At the same time, a call-in number was broadcast on US television, and Morgan Stanley officials even knocked on the doors of their employees.

The initial results were not good. "We didn't talk to as many people as you would hope," Mr Purcell said.

It took until Thursday evening before Morgan Stanley officials were able to say they had accounted for all but 15 of their employees.

Mr Purcell, his eyes welling with tears, said among the missing workers at his company were several "heroes" - employees who had stayed inside the building to help colleagues down as many as 74 floors to safety.

Morgan Stanley's plans to use its back-up facility were foiled at first. The company had prepared for an incident at the World Trade Center, but no one had imagined that all of lower Manhattan would be sealed off, leaving the facility inaccessible.

The situation in lower Manhattan began improving later in the week as city officials started more workers into their offices. By Friday morning, Morgan Stanley had moved 400 people into its back-up facility.

Assuming all goes well with computer systems, Morgan Stanley, like other securities firms, will begin trading stocks again on Monday.

But when it reopens, whenever that is, Morgan Stanley will be a different firm. Wall Street will be a different place. The Wall Street of old was a home for confident people who had known little other than success.

Wall Street knows tragedy now, first hand.