Most were turned back by officials of the Port Authority, which had commissioned and supervised the building of the centre in the Sixties.

Workers had looked across the divide after 8.48am at an inferno in their twin tower, and decided to make a run for it. They were told, however, to remain in their seats.

Others, who had made it down stairwells towards - or even out into - the lobby giving out on to the street and freedom, were told to run back upstairs, so that during the crucial 15 minutes of what should have been escape, there was confusion and a two-way rush along the panic-stricken arteries of life.

Meanwhile, the frantic rescue operation amid an apparently endless ocean of rubble continued yesterday, with 4,717 people now listed as missing. But for all their desperate efforts, and the long line of construction and former military volunteers waiting for days to join them, workers faced the grim reality that for many further people to be found alive under the original collapse would be a miracle.

In the flood of statistics that has followed the attacks on Tuesday morning, one stands out: Zero, the number of living survivors of the initial collapse rescued since Wednesday morning.

The last person to be found alive underneath the federal building in Oklahoma City was brought into the light 24 hours after the bomb that wrecked it.

The same is so far true of the World Trade Centre, with rescue workers pinning their final hopes on the system of train tunnels leading underneath the financial district, yesterday being pumped clear of flooding and groundwater.

The evidence that people were instructed by employers and security guards to remain in the south tower, and thus were condemned to death, is spreading this weekend.

Ernie Falk, who delivered packages to the offices of Morgan Stanley, the towers' biggest tenant, said that he was walking into the bank's reception area when he heard a 'horrendous boom' of the first plane's impact, and made a successful run for it.

But he told The Observer that as he ran downstairs, 'I heard people being told, "The building is secure. The safest place is inside; stay calm and do not leave". That's what they were saying. They were telling people to go back up to their offices and their desks, like the building was not in danger.'

A broker with the Morgan Stanley bank said that she had made it out on to the staircase and galloped more than 20 floors when she heard a voice on a megaphone telling her and others in flight to return and go back upstairs to the sixtieth floor.

The woman, who wished to be known as Eileen, said that she was consulting her lawyers with a view to taking the Port Authority to task, and would be in touch with Governor George Pataki - the nominal chief of the authority - to report what she saw.

The bank's employees duly went back upstairs, fighting a tidal flow of people from higher storeys running down, and sat at their desks.

When the second plane hit their own tower minutes later, at 9.03 am, 27 floors further up, all those remaining rose. Some went to windows and perished one way or another; others, like Eileen, ran back to the stairwell to try another dash, and some of those survived.

Another Morgan Stanley employee, Arturo Domingo, told the New York Times that he too had started down from the sixtieth floor, and was told to go back up by a man with a bullhorn. 'I really felt like punching that guy,' he said of the official, after returning to his office and managing a second escape following the second attack.

The announcements urging people to go back up were being made on the fortieth and forty-fourth floors, apparently by Port Authority officials.

People who worked in offices above the ninety-third storey would have been able to reach their workplaces only for the second plane to plough into the tower beneath them - leaving them with little or no chance of survival.

Any workers positioned on floors 87 to 93 would have been sent directly back into the path of the Boeing 767, and at the heart of the fireball.

One of those would have been Mary Thomas, had she obeyed instructions. Thomas worked in an architecture studio on the ninety-first storey, which was entirely taken out by the second plane. The scenes of confusion in the attempted evacuation of the second tower contrast markedly with those in the first, where the terrorists' plane hit higher.

There, the evacuation was orderly and fast, with people obviously managing to walk or run to safety from floors as high as the upper eighties.

Port Authority officials refused to comment on the reports about the south tower yesterday, and there was confusion about who was responsible for evacuation policy.

But the accounts were borne out by firefighters and rescue workers on Friday. They told The Observer unofficially that the majority of the bodies thus far extracted appeared to be towards the southern end of the tower complex - beneath the second tower to be hit, the first to fall.