An official investigation into the death of a prisoner at a detention facility in Shunping county, Hebei province, confirmed he died of an illness and not of unnatural causes as claimed by family members, local authorities said on Sunday.

The case has raised important issues over the state of the country's detention system.

Zhai Junbao, 29, was arrested for suspected theft in January, said Shi Xinsheng, secretary-general and spokesman of Shunping county government.