NARRATOR (JACK FORTUNE): 11 September 2001 began as a very normal day. Two hours later 2,800 people were dead and two of the most famous buildings in the world lay in ruins. This is the minute-by-minute story of what happened to the World Trade Center that day. It draws upon the testimony of eye-witnesses, experts and the official investigators and it is the first full scientific account of why the two towers collapsed. It is a story which may have major implications for anyone who builds, designs or even just goes into a tall building. Even today, Ground Zero has the capacity to shock because what happened here on 11 September still seems beyond comprehension.

BRIAN CLARK (World Trade Center South Tower survivor): He said "You know I think those buildings could go over." and I said "There's no way." I said "Those are steel structures." I didn't finish the sentence.

WOMAN (Screaming): Oh my God, oh my God!

NARRATOR: People now come as pilgrims to the site, to remember those who were lost and to try and understand how two of the world's tallest skyscrapers could have been destroyed so quickly. A disaster on this scale begs two crucial questions: was the collapse of the buildings inevitable, and need so many people have died?

MIKE MELDRUM (Ladder Six Fire Crew): I still find it hard to believe that these buildings are missing. I, I can't explain what happened, I can't explain how we walked out of that building.

NARRATOR: Some answers may still be found hidden in the rubble, but most lie in decisions taken 35 years ago when the towers were first built. It began in 1966 with a radical dream. The World Trade Center towers were designed to be more modern, more at the cutting edge and taller than any skyscraper ever built. They would launch a new wave in modern building methods. The head structural engineer on the project was Leslie Robertson, who was then just 34 years old.

LESLIE ROBERTSON (Leslie E. Robertson Associates): It was really a young person's project and it took a huge amount of energy. Did a lot of things that I don't think an older engineer would have, would have bothered to do because he would have had confidence in the work that he'd done in the past and I was charging down a different highway.

NARRATOR: All earlier skyscrapers, like the Empire State Building, used well-established methods. They were based upon a solid grid of girders. The result was structures of immense strength, but they all had the same drawback.

LESLIE ROBERTSON: The buildings of the past had columns spaces roughly 30ft on centre in all directions and the issue with that is, worked very well but it has columns and space that you would like to rent.

NARRATOR: To increase the useable floor space Robertson's radical idea was to reposition most of the inner columns to the outside so most of the load on the building was transferred to outer walls made of closely bound columns of steel. This steel outer skeleton took some of the downward weight of the building, but its main task was to handle the single biggest strain on a skyscraper - the force of the wind.

LESLIE ROBERTSON: That whole issue of wind engineering is the most important part of the structural design of any very tall building. It's just the brute strength of it is the driving force behind all structures of tall buildings.

NARRATOR: Though lighter than the traditional stone or brick, the steel outer skeletons were explicitly designed to do something all previous skyscrapers were not: they could actually bend in the wind, something that would be dramatically proven on 11 September.

BILL FORNEY (World Trade Center North Tower Survivor): It lurched back and forth. After maybe six to ten movements back and forth of that building it was over and it, it was still standing.

NARRATOR: To support most of the downward weight of the building Robertson created a separate inner core made of steel girders. The core also housed the lifts and emergency stairwells, but neither the outer skeleton nor the inner core could stand alone, so Robertson used steel floor trusses to knit the whole thing together.

LESLIE ROBERTSON: The World Trade Center is a very large project. In essence it still boils down to a series of small pieces and this is an example of a top part assembly of a typical floor truss.

NARRATOR: The floor trusses had a vital structural role. They held the towers firm bracing the outer skeleton against the inner core. Without the trusses the towers could not stand. Their performance is now at the heart of the investigation into what happened. Another area of innovation was in fire protection. To save weight the trusses were coated not in concrete but in the latest, lightweight, heat-resistant foam and instead of protecting the inner core with concrete the architects used both the spray and a lightweight fire resistant plasterboard called drywall. Drywall is very effective at keeping out fire, but it has one problem: it's not very strong.

BRIAN CLARK: Drywall had been blown off the wall and was lying on, you know propped up against the railing here and, and we had to move it, shovel it aside.

NARRATOR: The Trade Center's designers tried to anticipate every possible disaster. The towers were the first skyscrapers ever built explicitly to survive the impact of a plane.

LESLIE ROBERTSON: We had designed the project for the impact of the, our largest aeroplane of its time, the, the Boeing 707. That is to take this jet aeroplane, run it into the building, destroy a lot of structure and still have it stand up.

NARRATOR: But back when the World Trade Center was opened there seemed no need to worry. Everyone believed the twin towers to be as safe as any skyscraper in the world. In fact in 1993 they seemed to pass the ultimate test. Terrorists exploded a huge bomb underneath the World Trade Center complex. It blew a 90ft hole through five floors killing six people, but the towers stood completely firm.

LESLIE ROBERTSON: The bombing I think created a lot of confidence in everyone's mind that the Trade Center was pretty sturdy.

NARRATOR: On 11 September, 2001 at his desk in the North Tower of the World Trade Center, was transport consultant Paul Neal.

PAUL NEAL (North Tower survivor): The day was a beautiful clear day which I'm quite sure was significant because it meant that the, the hijackers of the aircraft would have had perfect visual conditions so they'd have been able to see those twin towers probably 60/70 miles away.

NARRATOR: Down below in the World Trade Center's underground station the morning rush-hour was just underway. It's now estimated that there were 14,000 people in the two towers at that time in the morning, far fewer than the 50,000 who would normally fill them later in the day.

FIRE-FIGHTER: Ladder Six, ladder Six only. Box 215, 120...

NARRATOR: Just a few blocks away, in New York's Chinatown, the Ladder 6 fire crew was going about its normal duties. Then one of them heard the roar of aircraft engines.

MATT KOMOROWSKI (Ladder 6 Fire Crew) We started pulling out quarters and I distinctly remember hearing from the dispatcher all low Manhattan units respond to the World Trade Center. I knew we were in for something big.

NARRATOR: Inside the stricken North Tower, just ten floors beneath where the plane had hit, was commodities trader Bill Forney.

BILL FORNEY: There was a high pitched scream, there was a, a tremendous change in the air pressure, the building lurched forward, back and forth. It was a scary situation, It was actually the first time that I had truly ever thought that I might die.

NARRATOR: An American Airlines 767 had been deliberately flown into the North Tower striking it across floor 93 to floor 98. The aircraft was swallowed up by the building as it hit at 440 miles per hour. At that speed the force of the impact was massive. The plane had created a huge void across six floors on one face of the North Tower. Two-thirds of the steel columns were severed, but the building stood firm. The weight of the tower was instantly redistributed around the hole. Leslie Robertson's radical design seemed to have worked, but there was more than just damage to the outer skeleton. As the aircraft carried on into the building its engines, each weighing four tons, slammed into the building's core. A fire then spread at extraordinary speed, all because of something the jet was carrying.

PAUL NEAL: Almost immediately after the impact somewhat bizarrely I smelt a, an overwhelming stench of aviation fuel, jet A1 gas which I recognised 'cos I'm a private pilot and I'm used to sort of airfield environments and I, and I recall, recall smelling it and almost instantly dismissed it as being illogical and didn't have any place in the World Trade Center.

NARRATOR: The jet fuel ignited paper, carpets and plastics. It spread a huge fire across all six floors where the plane had hit. The extent of the fire was something the designers had never anticipated.

LESLIE ROBERTSON: With the 707, to the best of my knowledge, the fuel load was not considered in the design and indeed I don't know how it could have been considered.

NARRATOR: The jet fuel caused the fire to spread so far and so fast that it effectively cut the building into two. For the 6,000 people below where the plane had hit the staircases still offered a means of escape, but for the 950 caught above the point of impact and the fire there was no way out.

LESLIE ROBERTSON: The people above, obviously they were suffering terribly, the people who elected to take their own destiny in their hands by jumping, I mean it must have been incredibly awful place above the impact.

NARRATOR: What seems to have happened is that the fire had flooded into the core and cut off all the stairways, something that should not have been possible, as they should have been protected by the fire-resistant drywall, but the problem seems to have been that the drywall was just not strong enough. One remarkable story illustrates just how weak the drywall was. 40 storeys below the crash site a lift jammed between floors trapping six people.

AL SMITH: If we don't get out of here will, are we going to suffocate in here, will the elevator move again? I think a lot of thoughts just raced to our mind at that particular moment.

NARRATOR: They forced the lift doors open and found themselves facing drywall. Jan Demczur attacked it with nothing more than his window cleaning squeegee.

JAN DEMCZUR: Whatever you have you have to try. Now in this particular time there was not brick or concrete or something, there was drywall.

AL SMITH: You take the handle of the squeegee, takes the rubber out to make a device to work with. I focus on this guy digging into the wall like there was no tomorrow.

JAN DEMCZUR: I was chopping I don't know my hand like was tight or something and I was putting and squeegee went straight through the hole and I lost my squeegee.

NARRATOR: Others in the lift took over and by kicking the drywall enlarged the hole. Al Smith, the thinnest, went through first.

AL SMITH: I was head first, then my shoulders, which was a tight squeeze, then I hollered back into the elevator for them to push my feet.

NARRATOR: It is a sad irony that it was the weakness of the drywall which gave the six in the lift their chance to escape because for those higher up that weakness may have proved fatal. It seems the drywall around the emergency staircases where the plane had hit were simply blown away, allowing the fire and smoke to flood in. That was almost certainly why nearly 1,000 people were trapped above the area of impact. Some building safety experts think that drywall, which is used in many modern buildings, is just too lightweight. A stronger fireproofing might have allowed many more to escape.

JAKE PAULS (Building Safety Consultant): If the stairs had been more hardened - let's put it that way - the walls would have been less able to be breached by the, by the collision of the aircraft. Perhaps one or two of the stairs would have survived the impact and that would have meant people from above maybe could have passed through the impact area.

NARRATOR: In the very early stages, just after the impact, all eyes were on the victims stuck high in the tower. No one was thinking that there might be even worse to come. But the seeds of destruction that would eventually bring down the North Tower had already been sown. First, the plane had dislodged, or destroyed, many of the floor trusses and those still in place had most of their fireproofing blown off. Most of the inner core columns seemed to have survived and could continue to carry the weight of the building, but their fireproofing, including the drywall, was pulverised. Without that protection the bare steel of the core was now exposed to intense heat.

MATTHYS LEVY (Structural Engineer): Steel will lose half its strength by the time it rea, reaches about 500 degrees Centigrade, so that fire caused the steel to soften up. The columns began to soften, buckle, fail.

NARRATOR: This baring of the steel to the heat was the first stage in the process that would eventually bring the tower down. Unaware of the danger that lay ahead, firemen prepared to enter the North Tower to fight the blaze, among them the crew from Ladder Six. They faced having to climb up staircases for 93 floors.

MATT KOMOROWSKI It was very slow. We, we took our time going up 'cos we have heavy gear. Very crowded. The civilians were coming down on our left in a single file.

NARRATOR: But the painful reality is that the fire-fighters' task was hopeless. The blaze was too high up and spread over too many floors. No equipment they carried would have made any difference. The structure was already starting to weaken. By now over 3,000 people had already managed to escape from the North Tower. It had just one and a half hours left to stand, but attention was about to shift to the South Tower. From the moment the first building was struck, many of the South Tower's 7,000 occupants had chosen to get out. That decision saved many lives, but not everyone went. Brian Clark stayed in his office on floor 84.

BRIAN CLARK: I am strictly guessing, but I would think we were perhaps down to about 25 people left on our floor. There was an announcement came over the system and said if you are in the midst of evacuation you may return to your office by using the re-entry doors on the re-entry floors and the elevators to return to your office.

NARRATOR: It's not known how many, but some turned around and went back up to their desks. Just five minutes later the terrorists struck again. A second hijacked 767 crashed into the South Tower hitting it between floors 78-84. Brian Clark's office was on the floor where the upper wing of the aircraft hit.

BRIAN CLARK: Our room fell apart at that moment. Complete destruction. For seven to ten seconds there was this enormous sway in the building and it was all one way and I just felt in my heart that oh my gosh, we're going over.

NARRATOR: The plane had sliced into the South Tower at an angle to the right. Unlike in the North Tower, it missed most of the core and the debris smashed along the eastern wall. Initially the South Tower responded well and seemed capable of standing, but in one vital respect it had been hit in a more critical place than the North Tower. It had been struck far lower down which meant the damaged structure was having to bear a much heavier load.

LESLIE ROBERTSON: And I kept saying to myself "What's going on inside? How bad is it inside?" and there's no way to measure it.

NARRATOR: Inside the tower the plane had again destroyed many floor trusses and stripped the fire protection off others. Though the core had not been hit so badly, much of its protective drywall had been shattered. Again stairwells and lift-shafts were exposed allowing the fire to spread. There were now about 2,000 people left alive in the South Tower, some 1500 below where the plane had crashed. Just as in the North Tower, they still had a way down, but for the 500 above the impact line two of the three staircases were completely impassable, but one still offered a way out.

BRIAN CLARK: So we started down that stairway and we only went three floors. There was a group of seven of us, myself and six others. We met two people that had come up from the floor 80, a heavy set woman and, by comparison, a rather frail male. She said stop, stop, you've got to go up and she laboured up to join us moving very slowly, she was such a big woman. She said you've got to go, you've got to go up, you can't go down, there's too much smoke and flame below.

NARRATOR: Clark then heard cries for help coming from an office nearby. It was banker Stanley Praimnath. Clark pulled him free and together they carried on down the tower, but their progress was hampered by one of the things that was meant to protect them: the fireproof drywall.

BRIAN CLARK: Drywall had been blown off the wall and was lying on, you know propped up against the railing here and, and we had to move it, shovel it aside. You could see through the wall and the cracks and see flames just, just licking up, not a roaring inferno, just quiet flames licking up and smoke sort of eking through the wall.

NARRATOR: Clark and Praimnath did make it through, but they were two of just four people who got out alive from above the impact zone in either tower. By now, less than quarter of an hour after it had been hit, all the causes of the collapse of the South Tower were in place. The huge weight of the top third of the building was bearing down on the damaged structure. Meanwhile, the outer steel columns were being softened by a fierce fire that was raging in the north-east corner. The fire would also have acted on the floor trusses. Some would have started to soften and sag pulling on the outer wall columns. The trusses were essential to hold the building together. If too many failed the building inevitably would become unstable.

LESLIE ROBERTSON: I think the structures were stalwart but they were not that stalwart. There was no fire suppression system that could even begin to deal with that, with that event, nothing, nothing, so I, I, I didn't know whether they would fall or not fall.

NARRATOR: The South Tower had now been burning for 50 minutes, the North Tower for over an hour. Office workers from both buildings were reaching ground level in a steady stream.

PAUL NEAL: There was burning debris over this whole layer, the plaza level were, well, bodies and body parts and I'm assuming these were the people who'd been jumping.

NARRATOR: Paul Neal headed out into the street. Others were guided into the concourse beneath the towers.

BILL FORNEY: They were ushering us forward, let's go, let's keep moving. We walked down these, these escalators, down to the tunnel system, the concourse underneath the World Trade Centers.

NARRATOR: Seven minutes later Brian Clark and Stanley Praimnath had made it down the South Tower and were four blocks away. They stopped and looked round.

BRIAN CLARK: And Stanley said to me, he said "You know I think those, those buildings could go over," and I said "There's no way," I said "Those are steel structures." I said "That's furniture and paper and carpeting and draperies and things like that that are burning."

REPORTER: This is as close as we can get to the base of the World Trade Center. You can see the firemen assembled here, the police officers...

BILL FORNEY: A tidal wave of, of destruction just flowed and I just remember tightening my eyes as tight as can be and grimacing, hoping that I wasn't going to die.

MAN: It's gone, the whole tower, it's gone. Holy crap. It knocked the whole frigging thing down.

DR GENE CORLEY (American Society of Civil Engineers): Large pictures like this and stop them, we...

NARRATOR: Since that day, investigators have poured over hundreds of hours of video images trying to find out exactly why the South Tower failed. The man in charge of the official inquiry is structural engineer Gene Corley. He noticed something in pictures taken by a nearby firm of architects. It shows that much of the core, which supported most of the downward weight of the building, was intact when the tower fell.

GENE CORLEY: It comes down. Looks like part of the core still showing there and the size and the spacing of the columns it looks like it must have been the core.

NARRATOR: IF so much of the core was standing the suggestion must be that it was not responsible for starting the collapse.

GENE CORLEY: Over here where the aircraft went in we can count the number of columns that...

NARRATOR: This footage also rules out the damage caused by the initial impact as the cause of collapse.

GENE CORLEY: Starts to collapse, spreads over to here in the top of the building now's on its way down.

NARRATOR: The building fell away from the side into which the jet had crashed. In fact the building fell towards the side where the fire was most concentrated and this suggests that a very particular mechanism was at work. The crucial damage occurred just after the plane smashed into the tower. It slammed along the eastern wall weakening the outer skeleton's steel columns on that side. It also began a fierce fire concentrated in the north-east corner. The theory is the heat of the fire softened not only the floor trusses but also the walls of the outer skeleton they were attached to. When the steel was sufficiently weakened by the heat the walls and the trusses would have pulled apart and without the trusses to hold them rigid the columns of the outer walls started to bend and then fail.

DR CHARLES THORNTON (American Society of Civil Engineers): As you start to lose the lateral support due to the floor the exterior just crumples like a piece of paper or a, like if you took a, a sheet of cardboard and you put some weight on it and you take out the lateral supports it'll just bow right out.

NARRATOR: This footage shows this process in action. A line of columns in the outer skeleton snaps. The top of the building then lurches outwards and falls. As it did so it would have dislodged many more floor trusses. The top floors must have then cascaded down onto the floors below with enormous force. The result is what is called a progressive collapse as each floor in turn pancaked down onto the one below. It took just 30 seconds for the tower to collapse entirely. In all, 600 people died in the South Tower. 500 of them were those trapped above the point of impact. It is impossible to overstate the shock of that first collapse.

PAUL NEAL: So I came back out onto the surface and came out into what would be my idea what a nuclear winter would have been like.

BRIAN CLARK: The building that I had worked in for 27 years was gone and that was just a staggering thought. I mean there was, there was then silence. People just couldn't believe it.

NARRATOR: There was now one terrible implication. If the South Tower had fallen, the other was likely to follow. An urgent message was radioed to all firemen and police in the North Tower: evacuate immediately. The Ladder Six team were then at floor 27.

MIKE MELDRUM: We heard somebody yelling on the radio it's time to start back down now.

MATT KOMOROWSKI We're trained to go and save people and go into dangerous situations and then when we're told to abandon our assignment it's a very odd thing.

NARRATOR: By now most of those who could get out had got out, but nearly 1,000 people were still trapped in the upper 20 storeys. 25 minutes later, with many firemen still inside the tower, including the team from Ladder Six, it came down.

MATT KOMOROWSKI: I felt an incredible rush of air at my back.

SAL D'AGOSTINO (Ladder Six Fire Crew): I remember hearing the boom of it, the boom. As the floors are pancaking I'm hearing that.

MIKE MELDRUM: It was like standing between two heavy freight trains in a tunnel going by you.

NARRATOR: Amazingly, all six members of the Ladder Six fire crew survived.

MIKE MELDRUM: At one point I said "Captain, this is a light above us." I thought it was somebody with a flashlight and I said "What is it?" and he said "Mick, there's a beautiful blue sky above us" (Yeah) and I said "Captain, there's a 105 storey building above us." He says "No", he says, "I think we are the top of the World Trade Center right now."

NARRATOR: In total, 2,800 people died in the attack on the World Trade Center. 479 of them were from the emergency services, 157 were on board the two jets. Most were office workers who had been trapped with no means of escape. Many experts now think the North Tower fell for different reasons to the South Tower. The main clue lies in what happened to the TV mast on top. It dropped straight down.

MATTHYS LEVY: It was very much like a controlled demolition when you look at it because the building essentially fell almost vertically down as if someone had deliberately set blasts to take place to cause the building to fall vertically downward.

NARRATOR: The mast was directly supported by the tower's inner core. The way it fell suggests it was failure of the inner core that began the collapse, whereas in the South Tower it had been the outer walls. The reason for this difference is probably the way in which the tower had been hit. The 767 had smashed through the outer wall and hit the inner core directly destroying the fire protection. The intense fire that followed had then concentrated around the core. Two things would then have happened. Floor trusses, softened by the fire, would have fallen away from the core. Without the trusses to hold it firm, the core would have lost crucial support. At the same time, the core's exposed steel girders, also long softened by the heat, would have begun to buckle under the weight of the tower. The result: another progressive collapse. Most of what remains of the World Trade Center is now in a scrap-metal yard across the river from Manhattan. Since 11 September the official investigators have been combing the wreckage looking for clues as to why the towers fell. In their minds is one crucial issue: the design of the World Trade Center inspired many other buildings. If there was something about it that makes skyscrapers especially vulnerable then there are implications for us all. Some of the investigators certainly have real concerns.

CHARLES THORNTON: A lot of people are saying that the structural engineering of the World Trade Center was miraculously wonderful, that the buildings stood up that long in, in, you know the case of two 767s flying into it. I would tend to think that they were not as successful as they could have been.

INVESTIGATOR: Some cladding up there that looks like...

NARRATOR: Thornton and his fellow investigators are certainly focussing on one issue in particular.

INVESTIGATOR: With the bar and angles...

NARRATOR: The role of the floor trusses and how they performed. The question is: did they fall away too easily? If they had been better protected and attached to the building more firmly might they have held the towers together for longer?

CHARLES THORNTON: Had the floor system been a, a more robust floor system with much stronger connections between the exterior and the inside I think the buildings, had they been a different floor design, probably would have lasted longer. Would they ultimately have collapsed? Maybe not.

NARRATOR: Others have questions about the fireproofing. The lightweight foam sprayed onto the steel girders and trusses is used all over the world, yet it seems to have been blown off with ease, leaving the steel exposed to the heat. And then there is the issue of the drywall. it is used as a fire protection in many buildings and yet it was not strong enough to withstand a window cleaner's squeegee, let alone a terrorist attack. Should something so fragile have been given the key role of protecting the core and the emergency staircases from fire? Had the designers used a stronger material might more people have survived?

MATTHYS LEVY: The core in concrete might have actually stood for a much longer period of time allowing many, many more occupants to leave the building. It would certainly have allowed the occupants on the upper floors to have a safe passage through at least one of the vertical stairwells. The core in concrete might have actually stood through the fire and survived and the building would not have collapsed.

NARRATOR: But others disagree with the critics. Among them the man leading the investigation. Gene Corley seems to think the buildings performed as well as anyone could reasonably expect.

GENE CORLEY: There certainly was the possibility of them collapsing immediately as each aircraft hit and the fact that one of them lasted 55 minutes and the other about an hour and 40 minutes to me says that they did very well.

NARRATOR: Corley is due to publish his interim report in April. It may well conclude that no skyscraper could have survived the assault of a fuel-laden jet, but until the issues are fully explored doubts about the safety of some tall buildings will surely remain. In the meantime, what is left is a fierce human tragedy and thousands of people trying to come to term with it.

BRIAN CLARK: We lost 61 dear friends that we worked with and laughed with for years and deeply saddened that they aren't here.

BILL FORNEY: I have visions. They happen to me during the day. I, I can sleep well at night, I don't have too many nightmares, but I have visions of things happening in front of me and all about me: a car bomb exploding, another building falling right in front of me, or with me inside it.

MIKE MELDRUM: I, I ride the ferry home at night and I still find it hard to believe that these buildings are missing. I, I can't explain what happened, I can't explain why anybody would, would go to that extent, I can't explain how we walked out of that building.

NARRATOR: But for Leslie Robertson, the man who built the World Trade Center, there is a special kind of torture. His office overlooks what was once his greatest achievement.

LESLIE ROBERTSON: Ground Zero is a very disturbing place for me. I mean I probably have more emotional attachment to it than maybe any other person now alive. I cannot escape the people who died there. Even if I'm looking down into a pile of rubble it's still to me somehow up there in the air burning and I cannot make that go away.