On 12 July 2015 a satellite swung over the rolling deserts and oasis cities of China's vast far west.
One of the images it captured that day just shows a patch of empty, untouched, ashen-grey sand.
It seems an unlikely place to start an investigation into one of the most pressing human rights concerns of our age.
But less than three years later, on 22 April 2018, a satellite photo of that same piece of desert showed something new.
A massive, highly secure compound had materialised.
It is enclosed with a 2km-long exterior wall punctuated by 16 guard towers.
The first reports that China was operating a system of internment camps for Muslims in Xinjiang began to emerge last year.
The satellite photograph was discovered by researchers looking for evidence of that system on the global mapping software, Google Earth.
It places the site just outside the small town of Dabancheng, about an hour's drive from the provincial capital, Urumqi.
To try to avoid the suffocating police scrutiny that awaits every visiting journalist, we land at Urumqi airport in the early hours of the morning.
But by the time we arrive in Dabancheng we're being followed by at least five cars, containing an assortment of uniformed and plain-clothes police officers and government officials.
It's already clear that our plan to visit a dozen suspected camps over the course of the next few days is not going to be easy.
As we drive up the wide approach road we know that sooner or later the convoy behind is going to try to stop us.
While still a few hundred metres away, we see something unexpected.
The wide expanse of dusty ground, shown on the satellite image to the east of the site, is empty no more.
In its place, a huge extension project is taking shape.
Like a mini-city sprouting from the desert and bristling with cranes, are row upon row of giant, grey buildings - all of them four storeys high.
With our cameras rolling we try to capture the extent of the construction, but before we can go much further one of the police cars swings into action.
Our car is stopped - we're told to turn off the cameras and to leave.
But we've discovered something of significance - a huge amount of extra activity that has so far gone unnoticed by the outside world.
In remote parts of the world, Google Earth images can take months or years to update.
Other public sources of satellite photography however - like the European Space Agency's Sentinel database - provide much more frequent images, although they're of a much lower resolution.
It is here that we find what we are looking for.
An October 2018 Sentinel image shows just how much the site has grown compared with what we'd expected to see.
What we suspected to be a big internment camp, now looks like an enormous one.
And it is just one of many similar, large prison-type structures that have been built across Xinjiang in the past few years.
Before our attempt to visit the site, we'd stopped in the centre of Dabancheng.
It was impossible to speak openly to anyone - minders lurked menacingly close by and would aggressively debrief anyone who even exchanged a greeting with us.
Instead, we telephoned random numbers in the town.
What was this large complex with its 16 watchtowers that the authorities were so desperate to stop us filming?
“It's a re-education school,” one hotelier told us.
“Yes, that's a re-education school,” another shopkeeper agreed.
“There are tens of thousands of people there now. They have some problems with their thoughts.”
This giant facility would of course fit no objective definition of a school.
In Xinjiang “going to school” has come to take on a meaning all of its own.
China has consistently denied that it is locking up Muslims without trial.
But a euphemism for the camps has long existed - education.
Almost certainly as a response to the mounting international criticism, the authorities have begun to double down on this description, with a full-on propaganda drive.
State-run TV has been showing glossy reports, full of clean classrooms and grateful students, apparently willingly submitting themselves to the coursework.
There is no mention of the grounds on which the students have been chosen for this “study” or how long the courses last.
But there are clues.
The interviews sound more like confessions.
“I have deeply understood my own mistakes,” one man tells the camera, vowing to be a good citizen “after I get home”.
The main purpose of these facilities, we're told, is to combat extremism, through a mixture of legal theory, work skills and Chinese language training.
That last item shows that whatever you want to call them - schools or camps - the intended target is the same.
The facilities are exclusively for Xinjiang's Muslim minorities, many of whom do not speak Chinese as their mother tongue.
The video suggests the school is operating a dress code - not a single one of the female students is wearing a headscarf.
Harsh new legal penalties have been introduced to curtail Islamic identity and practice - banning, among other things, long beards and headscarves, the religious instruction of children, and even Islamic-sounding names.
The policies appear to mark a fundamental shift in official thinking - separatism is no longer framed as a problem of a few isolated individuals, but as a problem inherent within Uighur culture and Islam in general.
It coincides with a tightening grip on society under President Xi Jinping, in which loyalties to family and faith must be subordinate to the only one that matters - loyalty to the Communist Party.
The Uighurs' unique identity makes them a target for suspicion.
That view has been reinforced by credible reports that hundreds have travelled to Syria to fight with various militant groups.
Uighurs are now subject to ethnic profiling at thousands of pedestrian and vehicle checkpoints while Han Chinese residents are often waved through.
They face severe travel restrictions, both within Xinjiang and beyond, with an edict forcing residents to surrender all passports to the police for “safe keeping”.
Uighur government officials are prohibited from practising Islam, from attending mosques or from fasting during Ramadan.
In 2002, Reyila Abulaiti travelled from Xinjiang to the UK to study.
She met and married a British man, took British citizenship and started a family.
Last year, her mother came for her usual summer visit, spending time with her daughter and grandson and doing a bit of London sightseeing.
Xiamuxinuer Pida, 66, is a well-educated former-engineer with a long service record at a Chinese state company.
She flew back to Xinjiang on 2 June.
Having not heard from her, Reyila called to check she'd got home OK.
The conversation was brief and terrifying.
“She told me that the police were searching the house,” Reyila remembers.
It was Reyila who appeared to be the target of the investigation.
She needed to send copies of her documents, her mother said - proof of UK address, a copy of her British passport, her UK telephone numbers and information about her university course.
And then, after asking her to send them via a Chinese mobile chat service, Xiamuxinuer said something that sent a chill down Reyila's spine.
“Don't call me again,” her mother told her. “Don't call me ever.”
It was the last time her daughter would hear her voice.
She believes she has been in a camp ever since.
“My mum has been detained for no reason,” she says. “As far as I know, the Chinese government wants to delete Uighur identity from the world.”
The exercise yard can clearly be seen on the satellite photo of the camp where he says he was held, in the oasis town of Hotan in southern Xinjiang.
“We sang the song called ‘Without the Communist Party There Can Be No New China,’” Ablet says.
“And they taught us laws. If you couldn't recite them in the correct way, you'd be beaten.”
He was there for a month in late 2015 and, in some ways, he is one of the lucky ones.
In the early days of the internment camps, the lengths of the re-education “courses” appear to have been shorter.
Over the past two years there are very few reports of anyone being released at all.
And since there has now been a mass recall of passports, Ablet was one of the last Uighurs able to leave China.
He has sought refuge in Turkey, a country with a sizeable Uighur diaspora because of strong cultural and linguistic links.
Ablet tells me that his 74-year-old father and eight of his siblings are in the camps. “There is no-one left outside,” he says.
For Uighurs outside Xinjiang news has almost completely dried up.
Fear breeds silence.
Reports of people being deleted from family chat groups, or told never to call again, are now commonplace.
Two of the things most central to Uighur culture - faith and family - are being systematically broken.
As a result of the detention of whole extended families, there are reports that many children are being placed in state orphanages.
Bilkiz Hibibullah arrived in Turkey in 2016 with five of her children.
Her youngest daughter, Sekine Hasan, who by now would be three and a half years old, stayed in Xinjiang with Bilkiz's husband.
She did not yet have a passport and the plan was that, when she got one, the family would reunite in Istanbul.
She never got that passport.
Bilkiz's daughter Sekine, whom she has not seen for more than two years
Bilkiz believes her husband was detained on 20 March last year.
She has since lost contact with the rest of her family and now has no idea where her daughter is.
“In the middle of the night, after my other children have gone to bed, I cry a lot,” she says.
“There is nothing more miserable than not knowing where your daughter is, if she is alive or dead.
“If she could hear me now, I'd say nothing but sorry.”
Using only publicly available, open-source satellite data, it's possible to shed light on Xinjiang's dark secret.
GMV is a multinational aerospace company with experience of monitoring infrastructure from space on behalf of organisations like the European Space Agency and the European Commission.
Their analysts went through a list of 101 facilities located across Xinjiang - drawn up from the various media reports and academic research about the re-education camp system.
One by one, they measured the growth of new sites and the expansion of existing ones.
They identified and compared common features such as watchtowers and security fencing - the kind of things needed to monitor and control the movement of people.
And they categorised the likelihood of each site actually being a security facility, placing 44 of them in the high or very high category.
Then they plotted the first detection by satellite of each of those 44 facilities over time.
Images show the extent of the building work that has taken place at part of the camp where Abdusalem Muhemet was held.
GMV cannot say what the sites are being used for. But it is clear that over the past few years China has been building a lot of new security facilities, at remarkable and increasing speed.
It is likely to be an underestimate of the true picture.
There is a striking conclusion - the recent trend is towards larger facilities.
The number of new construction projects this year has fallen when compared with 2017.
GMV calculates that, from this set of 44 sites alone, the surface area of secure facilities in Xinjiang has expanded by some 440 hectares since 2003.
This measurement refers to the whole site within the external security walls, not just the buildings.
But 440 hectares represents a lot of additional space.
For context, a 14-hectare site within the city of Los Angeles – containing the Twin Towers Correctional Facility and the Men's Central Jail - holds a combined total of almost 7,000 prisoners.
We took one of GMV's findings - the increase in building size at the facility in Dabancheng - and showed it to a team with long experience in prison design at the Australian-based Guymer Bailey Architects.
Using the measurements from the satellite images they calculated that, at an absolute minimum, the facility could provide space for about 11,000 detainees.
Even that minimum estimate would place it alongside some of the biggest prisons in the world.
Riker's Island in New York, the largest in the US, has space for 10,000 prisoners.
Silivri Prison outside Istanbul, often referred to as Europe's largest, is designed to house 11,000.
Guymer Bailey Architects (GBA) provided us with this analysis of the possible functions of the various buildings on the site.
Their minimum estimate for occupancy at Dabancheng assumes that the detainees are held only in single rooms.
If dormitories were used instead then the total capacity at Dabancheng would increase dramatically, GBA suggests, with an outer limit of about 130,000.
We also showed the images to Raphael Sperry, an architect and the president of the US-based organisation, Architects/Designers/Planners for Social Responsibility.
“This is a truly massive and bleak detention facility,” he told me.
“It appears as a place designed to pack as many people into as small an area as possible at the lowest construction cost.
“I think 11,000 is likely a significant underestimate... From the available information we can't tell how the interior is configured or what portion of the buildings is used for detention rather than other functions. Even so, your dormitory estimate of 130,000 people seems, sadly, quite possible.”
The lack of access to the site means there's no way of independently verifying this analysis.
We asked the authorities in Xinjiang to confirm what the site at Dabancheng is used for but have received no response.
Not all of Xinjiang's internment camps are the same.
Some of the secure facilities have not been built from scratch, but are conversions of structures previously used for other purposes, like schools or factories.
These are often smaller and located closer to the centre of towns or cities.
In the northern county of Yining we tried to visit a number of such camps.
We'd seen local government procurement documents for a project to create five “vocational skills education training centres” for the purposes of “safeguarding stability”.
In the centre of town we stop outside a large group of buildings that used to be Yining Number 3 Middle School.
A high, solid blue steel fence now surrounds the site and there's heavy security on the front gate.
There's a new watchtower by the playground and another one by what used to be the football pitch.
The pitch is now completely covered by six long steel-roofed buildings.
Outside, visiting family members are queueing up at the security check.
Once again, wherever we go in the town, two or three cars follow us.
When we try to get out to film at one of the camps, this one surrounded by a grey fence, we're stopped.
The officials, with hands over our camera lenses, tell us that there's important military training taking place in the area today and we're instructed to leave.
Outside the former school we see a family, a mother and two children, standing quietly by the fence.
One of the minders tries to stop them from talking but another appears to over-rule him.
“Let them speak,” she says.
I ask them whom they're visiting.
There's a pause, before the young boy answers, “My dad”.
The hands cover our lenses once again.
We leave Kashgar on the highway, heading southwest towards an area dotted with Uighur villages and farms, and a great many suspected camps.
We're being followed as usual but soon we run into an unexpected obstacle.
Ahead of us, the highway appears to have just been closed.
The police officers manning the roadblock tell us that surface of the road has melted in the hot sun.
“It's not safe to proceed,” they say.
We notice that the other cars are being directed into a car park in a shopping mall, and over the radio, we hear instructions to hold them there “for a while”.
We're told the wait could be four or five hours and we're advised to turn around.
We look for alternative routes, but another roadblock always seems to materialise, although the explanations change.
One road is closed for “military training”.
Four times, on four separate roads, we're turned around before we finally have to admit defeat.
Just a few kilometres away lies another giant camp said to hold around 10,000 people.
There are Uighurs in positions of authority in Xinjiang.
Many of the government officials and police officers who tailed and stopped us were Uighurs.
If they feel in any way conflicted they cannot say it of course.
But while the system of profiling and control has been likened by some to Apartheid, clearly that is not entirely accurate.
Many Uighurs do have a stake in the system.
In reality a better parallel can be found in China's own totalitarian past.
As in the Cultural Revolution, a society is being told that it needs to be taken apart in order to be saved.
Shohrat Zakir, a Uighur and, in theory, the second most powerful politician in the region, suggests the battle has almost been won.
“In the past 21 months, no violent terrorist attacks have occurred and the number of criminal cases, including those endangering public security, has dropped significantly,” he is reported to have told state media.
“Xinjiang is not only beautiful but also safe and stable.”
But when the detainees are released, what then?
The former camp inmates we spoke to were all burning with resentment.
And the world has yet to hear from anyone who has spent time in facilities like Dabancheng, the sinister and secretive facility of such immense proportions.
Our reporting adds to the evidence that the mass re-education programme is internment by any other name - the locking up of many thousands of Muslims without trial or charge, in fact with no access to any legal process at all.
China is already proclaiming it to be a success.
But history holds many troubling precedents about where such a project might end up.