REPUBLIKA.CO.ID, BEIJING -- Police in the Chinese city of Kunming said on Monday they would prosecute 20 people involved with the Fanya Metals Exchange, which authorities and investors say was a multi-billion-dollar Ponzi scheme.

After months of protests, dozens of investigators took over the Kunming-based Fanya exchange building late in 2015, and this year police arrested the head of the exchange, Shan Jiuliang.

In a brief statement on their official microblog, police in the southwestern city said that Shan, along with 19 others, had had their cases sent to the prosecutor for review on June 30, the next legal step in the process before they face trial.

It has not been possible to reach Shan for comment and unclear if he has been allowed to retain a lawyer.

Police said the investigation continued and they were working hard to recover stolen assets.

"(We) hope that investors can positively cooperate, and create a good basis for the further handling of assets connected with this case," Kunming police added, without elaborating.

Police said in June they had impounded more than 70,000 tonnes of non-ferrous metals and other assets of the exchange.

Launched in 2011, the Fanya exchange advertised itself as a state-supported organisation aimed at boosting prices of strategic metals mined in China.

It offered an investment product promising annual returns as high as 13.68 percent and the flexibility to deposit and withdraw money at will.

But it started restricting withdrawals last year, citing liquidity problems.

In July of last year, hundreds of people protested outside the exchange in Kunming, alleging it had lost investments of more than 40 billion yuan ($6 billion) and complaining of government inaction.

Local officials, however, have said they have done their job well.

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