Phelim Kine, Asia researcher for Human Rights Watch, told AFP news agency that the police investigation into Anyuanding Security Services was an "encouraging development".
But he said the case was only the tip of the iceberg.
"The fact is that the problem of black jails goes far beyond one company. It involves a web of government officials, security forces, huge numbers of plainclothes thugs and dozens of facilities in Beijing alone.
"Meaningful action against black jails will require the political will to locate and close all of them, freeing their detainees and prosecuting their captors."Read more...