As the twin towers burned on September 11, hundreds of people became trapped in narrow stairwells
For those who survived the initial impact of two passenger jets that slammed into the World Trade Centre on September 11, 2001, the nightmare was just beginning.
The simplest of escape routes were the stairwells.
To get out of twin towers, you had to take 2,071 steps to descend from the highest floor.
But as they desperately tried to flee the smoke and flames, some found themselves trapped in stifling, overcrowded stairwells, descending at an agonising pace.
"Once we hit [floor] 69, it was just ... a step a minute," survivor Arthur Lee would later recall.
The stairwells became a bottleneck as people trying to escape squeezed past firefighters running up towards the point of impact.
Hundreds of Americans on 9/11 who survived the initial impacts from the two hijacked aircraft perished for want of a safe exit.
The stairwells were a critical factor in the death toll that day. There were too few, too close together and with walls too weak to withstand the fire.
That nightmare scenario has furiously driven safety experts for the past two decades to push for vital changes to US building safety codes.
'This was all about money'
With many of the elevators no longer working after the jets hit, stairs became the only path to freedom for thousands trapped in the twin towers on 9/11.
Up or down, left or right, panicked choices determined the chances of survival for people, unaware the odds were already stacked against them in buildings designed to maximise profit, not safety.
Advocates for post-9/11 building safety have spent the past two decades trying to prevent another nightmare in stairwells.
"It's a lonely battle, fighting all these big, powerful groups," said Glenn Corbett, an expert in fire protection.
"Because money talks and it's big money."
Professor Corbett teaches security, fire and emergency management at the John Jay College of Criminal Justice in New York and advised the National Construction Safety Team that investigated the collapse of the World Trade Centre towers.
A volunteer New Jersey firefighter, he was driving into the city that morning when he saw the north tower burning and changed directions to get to his firehouse.
He told the ABC that since 9/11, he and fellow fire safety advocates had pushed for design changes in high rise buildings that should be "no brainers" for maximising evacuations from fires.
Time and again, they faced pushback from building industry groups, reluctant to give up valuable floor space.
"This was all about money. Space costs money," he said.
The flaw in the design
When the World Trade Centre opened in lower Manhattan in 1973, its twin towers were each 110 storeys high.
While they were under construction, New York City's building codes for high rises changed to allow fewer stairwells in the towers, halving the number required from six to three.
Keen to maximise open space without columns or other obstructions, the building designers placed the stairwells together in the same central area of the huge, 4,000-square-metre floors, around 20 metres apart.
When American Airlines Flight 11 struck the north tower at 8:46am it sheared through floors 93 to 99 and all three of the building's stairwells in this area were destroyed.
Hundreds of people above the impact site were trapped with no way out. They were killed when the tower collapsed.
At 9:03am, United Airlines Flight 175 crashed into the south tower, through floors 75 to 85, but this tower had a major difference, a 'sky lobby' around floor 78, with space to transfer between elevators and stairwells set further apart.
Here, one stairwell in the south tower survived the impact, and offered a vital means of escape for those on the upper floors.
Professor Corbett believes both towers should have held a fourth stairwell, based on building codes for occupancy rates, but the buildings' owner, the New York Port Authority, was exempt from complying with the city's building codes.
Port Authority documents showed an engineer's recommendation in 1965 that they "take advantage of the more lenient provisions regarding exit stairs".
To this day, Professor Corbett believes that decision "may well have cost lives".
"There would have been more stairs and the stairwells would have been placed further apart," he said.
The crowds in the stairwells
Decades before the tragedy of the World Trade Centre, public safety expert Jake Pauls was advocating for wider stairwells in high rise buildings.
Nicknamed "a warrior on egress" by fellow safety campaigners, Dr Pauls has more than five decades of experience in public health consulting, with a focus on stairway safety and usability in major evacuations.
Seeing the disaster unfold on 9/11, he wanted answers on whether more lives could have been saved.
"It lit a fire in my belly," he said.
Now 78, and still shuttling between Canada and the US working on safety committees, Dr Pauls told the ABC the stair width in the twin towers in 2001 was based on an antiquated measurement going back to pre-World War I standards.
The narrowness of the stairwell hindered both escape and rescue.
Building codes dictated that stairs must be at least 111.8 centimetres wide to theoretically allow two people to pass each other.
Pictures taken inside the stairwell show office workers in a devastatingly slow descent, at times stopped and pressed flat against the walls to allow firefighters carrying well over 20 kilos in heavy equipment to get up the stairs.
Evacuees included hundreds of people with physical disabilities, some in wheelchairs, being carried down by their co-workers through the narrow stairs.
Survivors said even as the descent moved at a snail's pace, people did not push.
"A man with a bloody face and bandage on his head walked by, followed by a woman who was hyperventilating," Billy Forney, a trader who fled the 85th floor of the north tower, recalled.
"No-one took advantage of the path they cleared."
Below the points of impact, the stairs saved thousands of people, with about 14,000 occupants of the lower floors making it out alive.
The death toll would likely have been much higher that day, but for one crucial factor: The time of attack.
Investigators estimate the buildings were only half full when the first plane struck at 8:46am.
Had the attack taken place later, the overcrowding they saw in the stairwells during the evacuation would have been catastrophic, with thousands more trapped as the towers collapsed.
Why some people never attempted to escape
More than 400 first responders died on 9/11, including firefighters running into the stairwells and buildings unable to see or hear what was happening inside, and racing headlong to their deaths.
There were no video cameras in the stairwells and radio communications had broken down, leaving them unable to coordinate rescue efforts or receive warnings to evacuate.
Accounts from survivors paint a grim picture of how the vacuum of information inside the towers compounded the death toll.
Even with the sole stairwell in the south tower remaining passable, the occupants above the point of impact didn't get the information they needed to make the right choice.
When emergency operator recordings were released of distress calls that day, large numbers of victims had called for help from their mobile phones, only to be told to stay put and "defend-in-place".
The overwhelming majority complied.
Professor Corbett believes that if information from survivors who had made it down the stairs had been relayed to those still in the building with a quick call to their phones, many more may have made it out alive.
Are buildings now safer from attacks?
The US approved 23 building and fire code modifications in 2008, following investigations into the World Trade Centre disaster.
But safety advocates view the changes as a mixed success.
They included measures to improve fire resistance in building materials, to reinforce structures against collapse, and add blast-resistant walls to elevator and stairwell shafts — all designed to help buildings stay intact long enough to get people out.
"We can't design for all scenarios but we can design buildings in a way that buys time," said Karl Fippinger from the International Code Council (ICC), a non-profit that develops building codes in the US.
High-rise buildings were required to improve radio coverage systems to ensure emergency crews can communicate with each other inside, and with personnel outside.
Dr Pauls' proposal for video cameras in stairwells was not adopted.
A requirement for an extra stairwell did get through, but only in buildings above 128 metres, more than 40 storeys high.
"There's nothing magical about that number and those safety features could and should be looked at for smaller high rises," Professor Corbett said.
The width of stairways would be increased by 50 per cent, but only in a building code that does not cover most of the new high rise buildings being built across the United States today, including New York City.
While ICC codes are broadly adopted around the US, they are a minimum standard for building and fire codes and it's up to states and local jurisdictions to decide what to enforce.
Asked whether today's modern buildings could better withstand an attack like that inflicted on the twin towers on 9/11, Professor Corbett is unsure.
"Our old-school high rises like the Empire State Building are built like the Rock of Gibraltar," he said.
Karl Fippinger said the ICC will continue pushing the building industry to go above minimum safety codes.
"Twenty years later, building codes do reflect the sacrifice of the people who went before us, who perished on 9/11," he said.
"We owe them a debt of gratitude for their sacrifice."