Thirteen people have been detained for allegedly selling more than 4 million pieces of private information, police in north China's Hebei Province said Wednesday.

The case surfaced when a man named Wang from Hengshui City received many unsolicited calls at the end of 2016 and thought his personal data may have been stolen.

In March, he found two online social networking groups suspected of trafficking private data and purchased 40,000 pieces of personal information for 500 yuan (74 U.S. dollars) as evidence, and reported the matter to the police.

Police located two suspects in Shijiazhuang and Hengshui and took them into custody in late May.

Further investigation revealed a gang of 11 in Henan, Hubei and Shandong. Over 4 million pieces of data were seized, involving victims across 10 provinces.

Investigation showed the two suspects in Hebei had worked at the same company and started to traffic personal data after one of them quitted.

They sold information for less than one yuan a piece to the eleven suspects, who then sold it on to other companies.

Under the Criminal Law, those convicted of selling or providing personal information could face up to seven years in prison if "the circumstances are especially serious."

According to the new judicial interpretation, situations considered "especially serious" include illegally obtaining, selling or providing 5,000 or more items of communication records, accommodation, health or transaction information.

The investigation continues.