Thanks for downloading this edition of assignment from the BBC World Service.

I'm John Sudworth.

The Chinese government stands accused of incarcerating more than a million Muslims in a new system of massive detention

camps. It insists they are schools. I'm looking for the truth in a part of

China where people are too frightened to talk and

where journalists are far from welcome.

This week's assignment is the story of a search for something the Chinese authorities do not want us to find.

With reports of hundreds of thousands of missing Muslims,

this is the story of the evidence we've found despite the attempts to stop us.

Why can't we ask them questions?

Join me, John Sudworth here on BBC World Service,

as I search for China's hidden camps.

Xinjiang, China's vast far western region is a place of towering mountain rangers and wind-swept rolling sand dunes. It's a place of stark beauty and of fear.

Checkpoints control the desert highways and the Oasis towns and cities bristle with

security armored trucks flanked by police cars patrol constantly.

These public displays of strength are designed, the authorities say, to face down the threat of terrorism.

And there's something else here too,

something they don't want the world to see.

So that black one without the number please definitely yet. There's a silver one.

So it's half past 8 in the morning now and we've just arrived in the city of

Eaning and straight away as we left the train station.

We've got one, two, and another one back there, three cars following us.

...