The managers know this. They know, too, that, as a female supervisor, Shapla is trusted more than her male counterparts, and not just by “her girls”. So when they want the workers to stop shouting and calm down, and to enter the building, take their places at their machines and finish the big orders the factories are currently under pressure to complete on time, several come to her, telling her again how the building is safe, how the girls trust her, how if she doesn\\\’t go in, the girls won\\\’t either. She is uncertain, reluctant, but because at that moment it is the easiest thing to do, she gives way and goes in. And her girls follow.

Similar pressure has been put on other supervisors. They, too, reluctantly head into the building. Soon all the workers are flowing up the stairs. Mahmuda leaves her husband in the stairwell. He carries on up to his factory. She makes her way to her desk and machine on the third floor. On the fifth floor, in the Ether Tex factory, 15-year-old Preity, her mother and her sister are working by 8.30am. Preity is on her feet, moving up and down the line, clearing offcuts, bringing new needles or thread; her mother is nearby, her sister working at a sewing machine on the same line but at the opposite end of the room.

Mahmuda has time to stitch maybe two or three pieces before the lights go out, the old fans, which barely dent the heat in the factory, slow and the sewing machines stop. A power cut, frequent enough and no cause for alarm. The workers wait in the gloom, talking quietly, worried, waiting for the powerful, heavy generators installed on the roof to start up.

Moments after the generators start, sending vibrations through the building, a pillar in one corner of the Rana Plaza gives way with a loud, explosive bang. Then each storey slides sideways, tips and splits, falling in on the one below. On the third floor, the collapse is almost instantaneous, but the workers of Phantom Apparels still have time to realise what is happening.

The floor starts shaking and it is clear the building is coming down. Mahmuda starts reciting verses of the Qur\\\’an. She staggers, falls and crawls beneath her sewing machine.

Shapla is walking down her line when the building starts to collapse. She starts running. She, too, seeks protection under a sewing machine and then everything gives way around her. She is in darkness, gasping for breath.

Two floors above, the floor inclines slowly like the deck of a sinking ship. Panicked workers rush to the two narrow exits. It is dark, there is much dust and noise. Runarini has managed to find her youngest daughter, Preity, and is now trying to get to the other end of her line where Shamapati, her eldest, was working at her machine. But the force of bodies pushes her towards the exit. She cannot hold on to Preity and fight the crowd to find Shamapati. The floor lurches, tips again and everything falls.

In the darkness after the collapse there are many voices: sobs, sustained screaming, calls for help and water, moans of pain, prayers, howls of grief. Some are trapped in total darkness, others can make out some light. Many are pinned down by huge blocks of concrete, bent iron girders, machinery. Others are entombed in small cavities. Some are alone, others with colleagues or strangers from other floors. Shapla can move her hand but nothing else. Mahmuda can see the sky through a gap in the wall several yards away. Runarini and Preity are trapped together and they can hear voices, but not Shamapati\\\’s. They shout her name, but there is no response.

Outside there is chaos. Dazed survivors stand immobile in a huge, roiling cloud of dust. It takes time for Dhaka\\\’s ramshackle emergency services to arrive, so hundreds of locals clamber over and through the rubble, tearing at the concrete blocks and mangled metal with their hands. Soon corpses are lined up on the ground, limbs limp and twisted, as if they had fallen from a great height. Mahmuda crawls towards the light, finds herself only a few metres from the ground and clambers down. She cannot see her husband.

Shapla waits longer for rescue. She can hear sirens and shouts outside. They are hammering and drilling the slab of concrete above her. She finds her throat so dry from screaming that she cannot talk when the rescuers find her. She is unaware that she has made any noise whatsoever. It takes two hours to free her.

Runarini and Preity crawl together towards a shaft of light and are lifted from the rubble by mid-afternoon. The ruined building is now surrounded by police and soldiers, and heavy lifting equipment is arriving. There are electric saws and jackhammers, and lines of ambulances. There is no sign of Shamapati.

Bangladesh is the original “basket case”, a term coined by Henry Kissinger, the US secretary of state, to describe the country immediately after its violent secession from Pakistan in 1971. Up to 3 million died in that conflict, a civil war that was simultaneously a war of independence, and its wounds remain livid. Famine, flood, a massive programme of nationalisation, political instability and further violence crippled the new nation\\\’s economy. But, despite the continual challenges, the textiles and garment industry prospered. Investment and advice from Korean companies helped and, by the end of the 1990s, the industry had somehow weathered destructive storms – political, economic and meteorological.

In 2004, when the protectionist quotas that had been imposed to protect western jobs 30 years earlier expired, many in Bangladesh were apprehensive. But the industry boomed. “Business just took off,” says Rubana Huq, now managing director of garment manufacturers the Mohammadi Group. In 2004 there were 2 million workers in Bangladesh\\\’s 4,000 factories, with exports worth $6bn. Nine years later, there were twice as many in 5,600 registered establishments, sending $21bn worth of clothes overseas.

Competition from abroad when the quotas disappeared meant “prices dipped”, Huq says, but the demand from Europe and the US was so great that she and others “just kept on building”. If wages in Bangladesh remained the lowest in the world, land prices in central Dhaka, where most of the garment businesses were based, rose so fast that new investors sought space on the margins of the city where hastily reclaimed wetlands could still be bought relatively cheaply. In Savar, Gazipur, Ashulia and elsewhere around Dhaka, hundreds of factories went up every year. There was little or no planning or regulation.

“People were not much concerned by building codes or quality of material,” admits Emdadul Islam, the long-serving chief engineer of Rajuk, Dhaka\\\’s development authority. Monitoring of environmental impact, construction quality and permits for the millions flowing in from rural areas were almost nonexistent.

Inevitably, people started to die. In April 2005, a factory in Savar called Spectrum collapsed, killing 64 workers. The dead were making clothes for western retailers. Poor cement was a likely cause – builders often charge for better-quality materials than they deliver – or water from a nearby canal may have washed out foundations. For a short period there was talk of a crackdown on unsafe buildings, but even though a second factory in northern Dhaka collapsed a year later, killing 21, the fuss faded. As the recession hit western economies in 2008 and 2009, brands forced down prices even further as they negotiated with suppliers. Those in Bangladesh who demanded government intervention in one of the country\\\’s few economic success stories made little headway when dozens of garment factory owners sat in parliament and powerful industry bodies had the ear of policymakers. The boom continued. Bangladesh, the world\\\’s 76th biggest exporter of clothes in 1980, was the eighth biggest in 2006, and by 2013 was second only to China.

Over the decade, prompted by a string of reports of child labour and other abuses in factories around the world, the brands had set up systems to monitor pay and working conditions in their supply chain. Some organised their own inspections, many brought in contractors. But executives did not think to undertake structural surveys of the buildings where their clothes were made. Instead, they relied on the corrupt, poorly paid, underqualified, undermanned local authorities. This, senior executives at major European brands now admit, was a mistake. Others use stronger terms. From 2005 to March 2013, fires killed an estimated 600, but no more buildings collapsed.

The Dhaka suburb of Gulshan is only 15 miles from Savar. Rickshaws negotiate its quiet, tree-lined streets with relative ease and there are many good restaurants. In the late morning, coffee shops are busy with staff from international NGOs and the UN. This is where the Bangladesh operations of many big European and US brands are based, as are the larger buyers who often act as their intermediaries locally.

Fifteen-year-old Preity and her mother managed to escape from the building; her 18-year-old sister Shamapati didn’t make it out. Photograph: David Levene for the Guardian

The offices of H&M occupy a new five-storey block. There is nothing to distinguish the building, inside or out, from its counterparts in any western capital. On the walls of brightly lit meeting rooms – each named after garment manufacturing zones around the city – are posters of laughing, thin, beautiful young Europeans of varying ethnic backgrounds wearing the bright, cheery, fashionable clothes of the company\\\’s brands. A notice tells staff: “Your passionate focus towards quality will make it happen.”

H&M, which has 3,000 stores in around 50 markets worldwide and sales of £13bn in 2012 (making a profit of £1.6bn), has been in Bangladesh since the early 80s and is one of the biggest foreign brands working in the country. The Swedish retailer did not have any contracts with the factories working in Rana Plaza, nor any link to the building, and has long been at the forefront of efforts to find a way to improve pay and working conditions in the industry. These have intensified since last April\\\’s tragedy, but “there\\\’s a lot more to the country for us than low wages and low costs”, says Anna Gedda, the company\\\’s social sustainability manager.

Executives from other big brands all make similar points when talking about the Rana Plaza collapse, even if their companies have been less proactive than H&M. They express their shock and grief, say they have long recognised the problems in Bangladesh, point out that the chaotic environment of a developing world city means many issues are partly beyond their control (though, they admit, still their responsibility), and downplay the significance of cheap labour in the decision to locate production in the country. They also emphasise the importance of the garment industry to the Bangladeshi economy and the social transformation brought about by the employment of millions of young, rural women, albeit in poorly paid, monotonous jobs. One company spokesman points out that otherwise “these women would be in the fields, in ship-breaking or shrimp farming, working as maids”. Now, he says, they are breadwinners, independent, and often with the means to pay for their kids to go to school.

H&M\\\’s Gedda says: “The best way for the country is for brands like H&M to stay there.” This is something one hears frequently: from the Bangladesh Garment Manufacturers and Exporters Association, senior ministers, international trade unionists, independent economists and urban development specialists in Dhaka. It\\\’s also what all the workers say. Everyone agrees that the Rana Plaza collapse was a “wake-up call” that will be “a turning point”, but they all say the industry and the brands must remain. “If there is a pull-out, it will be a catastrophe for Dhaka,” says Professor Nazrul Islam, of the local Centre for Urban Studies.

The result, after a series of fairly acrimonious meetings in Geneva and Paris last year, is three separate initiatives: the Accord, which involves more than 150 largely European brands; the Alliance, set up by US brands; and a joint effort by the UN and the Bangladeshi government. Between them, these three are meant to bring all the factories in Bangladesh into a system of auditing and inspection that will enforce agreed standards. They will also, theoretically, ensure structural surveys. The brands will help pay for improvements needed by their suppliers to meet the new standards – through grants in the case of the legally binding Accord and soft loans for the less constraining Alliance – and there are also provisions for strengthening the country\\\’s underdeveloped and highly politicised unions. There is a separate process designed to establish how much compensation will be paid – the total could be more than $40m – to the Rana Plaza victims. Many brands have refused to become involved and international trade unionists admit the current initiatives have many flaws. Nonetheless, they say, taken together the package represents “an unprecedented chance to put all this right”.

Some specialists even hope that initiatives will provide a model not just in Bangladesh but in Cambodia, Indonesia, Latin America and even the emerging new markets for production such as Burma and Ethiopia. The risk, of course, is that attention will fade, as it did after the 2005 Spectrum factory collapse. “This is Plan B,” says Srinivas Reddy, the Bangladesh country director of the International Labour Organisation. Plan A led to Rana Plaza. There is no Plan C.

If, from afar, the world of the garment workers and the garment industry in Bangladesh is one of stark contrasts, up close it is one of shadows and ambiguity. Mohammadpur, for example, is a designated slum that is also a booming new neighbourhood on the western margins of a city that has no legal boundaries. A few decades ago it was wetland like Savar or Gazipur. By the 90s, the marshes had been replaced by rows of wooden sheds on stilts full of half-starved labourers and their families. Now it is changing again as Dhaka, a city of at least 15 million, gets steadily less poor. The wooden shacks have been replaced by cement apartments and rooms. There are tea stalls with TVs tuned to the Discovery Channel, schools, shops and a refurbished mosque. “It\\\’s better than where I came from… At least there\\\’s a chance of making a living here, of a better life, perhaps,” says Salahuddin, a 27-year-old labourer who fled poverty, debt and failing yields on a tiny parcel of land to bring his family to the city.

But if it is possible to see incremental and uneven progress in Dhaka, there is also misery, overcrowding, disease, crime and failing infrastructure. Police rarely venture into Mohammadpur. A fire in a garment factory here killed seven in 2013 and fear of such accidents is one reason Salahuddin\\\’s wife, Juleika, prefers poorly paid work as a maid. The dirt roads are strewn with rubbish. Water from standpipes is sold at extortionate prices by landlords in league with local politicians and bureaucrats. There is the perpetual risk of eviction and demolition. In the brutal summer heat, the concrete boxes where entire families live are furnaces.

Then there is Bangladesh. It is one of the “Next 11” nations seen by Goldman Sachs as having the potential to take off as major economies. Its development indicators have risen steadily, to the point where most are now better than those in neighbouring India. But again, if the rate of population growth has dropped in recent years, it is still high. Flooding and extreme weather events linked to climate change threaten disasters in the future in this low-lying land. The flow from the countryside to the cities is unlikely to slow soon, and, as the effluent from the factories contaminates the once pristine waterways, so toxic politics and endemic graft poison public life.

Of 20 women Shapla supervised in the building, 19 died. Photograph: David Levene for the Guardian

And what of the garment industry\\\’s role in all of this? Along with remittances from workers overseas, the industry is the only major foreign currency earner for the country. Wages for workers have risen since the disaster – the minimum monthly salary for an entry-level worker is now 5,300BDT (about £40) a month – but so have rents, and no one can pretend a family can live on a single salary. Factory conditions might have improved in the last decade, but even without the 1,130 who died in Rana Plaza, the toll from fires and other accidents would be intolerable. Along with the 4 million directly employed by the industry, economists say there are another 4 million people whose livelihoods depend on the internal demand it generates. If half of these 8 million people support a family of five, then around a sixth of the population depends on people several thousands miles away buying cheap clothes that they don\\\’t need.

And that means this world of ambiguity is one the consumer inhabits, too. Many debate where the blame lies for tragedies such as Rana Plaza, and argue over who has the responsibility to prevent them in the future. The owners of the factories? Those who let the buildings that house the workshops? The crooked builders? The bent bureaucrats? The government? The intermediaries who do so much of the buying? The brands?

Some western brands implicitly accept a degree of moral responsibility, even if their lawyers are careful to make sure their legal liability is strictly limited. “People portray companies as evil by definition, and in many aspects they are. But often times a company is forced to behave in certain ways by consumer behaviour. If no one buys your product, you are out of business. Consumers take decisions for companies, but often [they] don\\\’t, which means they don\\\’t care,” says one senior executive at a big European high street brand.

Though there is little statistical evidence of any direct correlation between bad publicity about social issues and sales – it would, experts say, be almost impossible to establish such a link anyway – big companies do their own research to understand the impact on sales of events such as the Rana Plaza collapse.

“Companies do very precise work on this to understand the consumers and what level of quality they are willing to pay for, and [it shows that] a large number of consumers prefer inexpensive over respect for human rights or environment,” the executive says. “After all, if you buy a pair of jeans at $9.99, what are you really expecting about the working conditions of those who made them or even just the environment in which they live?”

Mahmuda eventually found her husband\\\’s body, intact but for a broken nose, laid out in the makeshift morgue set up in the grounds of a local school. It took some days for her to realise that her husband was not going to walk through the door of their small home in Savar. She returned to her village to bury him, but life was as hard there as it had ever been and she was soon back in Savar. Eight months later, Mahmuda started work at another garment factory, less than 300m from where the Rana Plaza complex stood. She passes the site of the collapse – the rubble scraped away, a trough of filthy water, scattered bolts of filthy cloth, a stench of decayed matter – every day. “I tell myself if one building has collapsed, it doesn\\\’t mean they all will,” she says. “I can\\\’t be scared. If I am scared, how will I feed my family? I tell myself if one building has collapsed, it doesn\\\’t mean they all will.”

Runarini, however, has still not fully understood that her daughter will not come home. Shamapati\\\’s remains have never been found. It is possible that the 18-year-old\\\’s corpse may have been buried with 250 others, all unidentified, in the chaotic aftermath of the collapse. There is even a chance that, unrecognisable and untested, it may have been given to the wrong family. Bangladesh\\\’s only DNA testing facility was unable to cope with so many dead, and officials admit mistakes were made.

Preity still dreams of her village and misses her friends and school. Now she hopes to set up a tailoring shop when she is older, with help from an NGO. “My mum cries all the time,” she says. She misses her sister, too, and Shonjit, who sometimes walked her back from the factory. He was killed, too.

When Shonjit\\\’s father, Satyadip, who still works as a loader in a nearby factory, heard of the collapse, he ran to the site. “I was crazy. I tried to tear away the stones with my hands. For 13 days I went there, round all the hospitals, to find him, to find my son. I held on to my hope, but then we found him and he was not with us any more.”

Rubana Huq, managing director of the Mohammadi Group, travelled to the site of the collapse when she heard what had happened, though she has no link to the factories there. She set up a small relief operation and spent days handing out water and food to rescue workers. “It made me think differently about the workers; that it is another life, like mine,” she says.

Shapla has seen only one of the 20 girls she watched over since the day of the tragedy. She saw her, fleetingly, at a bus stop. She has heard news of a second. The rest she believes are dead. Now, when she sees their children, she feels “like a criminal”. “I was the one who got them to go in. I was the one they followed. I think about it all the time.”

Shapla has found a job at another factory but, due to her back injuries, as a sewing-machine operator, not a supervisor. She would prefer to do “any other job”, but the only alternative is manual labour on construction sites, building new factories and homes for workers, and she needs 10,000BDT (£75) to get her son into a local school.

Most of the victims have received money – some from the government, some from industry bodies, some from one or two brands, such as Primark, that have unilaterally decided to make payments while most other brands, the government, global unions and industry bodies argue over the level of compensation they will eventually pay out.

Every evening, Mahmuda, Satyadip, Shapla and approximately 4 million others in and around Dhaka stop work, exchange a few words with their colleagues and head home. They go out on to the broken, crowded streets, a stream of workers that flows together, then divides again round the stalls, the beggars, the lines of overloaded trucks, the packed buses immobile amid the rickshaws. They are joined by streams from other factories, flowing together before splitting again into smaller streams down the narrow lanes between the tenements, until these, too, eventually divide and divide once more, and it becomes dark, and the workers are home.

 We Are What We Wear: Unravelling Fast Fashion And The Collapse Of Rana Plaza by Lucy Siegle and Jason Burke is published by Guardian Shorts (ebook, £1.99/$2.99). Visit guardianshorts.com to read an extract and buy your copy. The Shirts On Our Backs, an interactive documentary about Rana Plaza and the textile industry, is on theguardian.com/rana.