China’s vast security apparatus shrouds itself in shadows, but the outside world has caught periodic glimpses of it behind the faded gray walls of Shijiazhuang prison in the northern province of Hebei.

Chinese media reports have shown inmates hunched over sewing machines in a garment workshop in the sprawling facility. Business leaders and Chinese Communist Party dignitaries have praised the penitentiary for exemplifying President Xi Jinping’s views on the rule of law.

But the prison has an alarming secret, U.S. congressional investigators disclosed last year. They revealed evidence showing that it is a Chinese government outpost in the trafficking pipeline that inundates the United States with fentanyl.