There are so many many sources but here is a good summary from NyTimes: http://archive.is/93x6g
Quotes from article under each line item definition. Again so many more sources but that would be exhausting to compile.
- Causing serious bodily or mental harm to members of the group
"Those who have been held in camps describe a rigorous prison environment filled with monotonous political indoctrination and, for many, terrorizing bouts of violence and physical abuse by guards."
-Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part
"Officials have held one million or more people in internment camps in Xinjiang, the country’s most sweeping mass detention program since the Mao era. A wide range of behavior can lead to detention, including acts of religious devotion, travel to certain countries, violations of birth restrictions or installing cellphone apps that allow encrypted messaging."
"And the authorities have carried out the widespread destruction of mosques and shrines while turning others into tourist sites."
- Imposing measures intended to prevent births within the group
"has used repressive methods such as forced sterilizations."
- Forcibly transferring children of the group to another group
"Large numbers of children in Xinjiang have been placed in boarding schools designed to assimilate and indoctrinate them, according to the government’s published plans."
- Killing members of the group
Article doesn't have a quote and there isn't a lot of evidence here so it is likely not happening on a large scale. Though there are many deaths from poor treatment in concentration camps and forced labor camps.
So why doesn't China allow independent investigations into what's really taking place? Maybe if they weren't hiding the truth people wouldn't be so quick to fill in the holes with their own stories.
Why they should? The burden of proof is on the accusers. "Independent" investigation guarantees nothing. Eg WMD in Iraq.
Remember the independent investigators in Iraq and how mocked they were in order to manufacture consent for the invasion after they found no wmds?
Yeah, you might not, but the chinese leadership does remember that
Edit to add: Downvotes don't make my statement any less true.
Insofar as how long it goes on, if you look at US WWI propaganda they were talking about German soldiers massacring babies in Belgium (the killing babies lie is a common one - from incubators to Assad gassing babies)