Many victims of a new form of debt trap designed by loan sharks who lend money at exorbitant interest rates with the aim of taking possession of their targets' residences, can look to Zhang Chen, a 45-year-old policeman in Shanghai, for help.

Zhang, who has been doing criminal investigations for more than 25 years, first identified such a trap in 2016 when a woman reported that she had borrowed 200,000 yuan ($29,500) from a financing company but was forced to pay 600,000 yuan on the second day after the due date, and later another 70,000 yuan to redeem her note.

But the preliminary investigation revealed signed contracts and bank statements - later determined to be forgeries - indicating that the victim had received 600,000 yuan, which made the case a civil one.

Zhang, however, remained doubtful.

"The evidence was so well-prepared, more than one would normally expect," said Zhang, leader of the Shanghai Public Security Bureau's third criminal investigation team.

Zhang researched previous cases related to the company and found another two victims with similar experiences. Both ended up being taken to the court and had their property frozen because of the "solid evidence" provided by the company.

He questioned the two victims, and was able to prove that the evidence provided by the company's attorney was forged.

"While this kind of fraudulent loan scheme looks legitimate - as some criminal gangs even hire professional attorneys - it is dangerous. It is designed to bankrupt the borrowers, who have to repay loans as small as 50,000 yuan with an apartment worth millions," Zhang said.

Shanghai has since begun to crack down on such loan schemes, which target juveniles in most cases. As of the end of October, the city had nailed 316 criminal gangs, detained more than 1,770 suspects and retrieved more than 1.2 billion yuan in losses for the victims.

The city's successful control of such crimes has yielded a model that could be followed elsewhere.

"Such a case is usually hard to define. But once you see through its essence, which is fraud and extortion, it reveals itself despite the various forms and methods it has adopted to disguise its fraudulent nature," Zhang said.

It is important to protect citizens' personal interests and property, Zhang said, adding that the fraudulent loans affect the credibility of the government, courts and society. "We have to stop them," he said.

Zhang, whose team also serves the Shanghai Liaison Office of the Interpol National Central Bureau in China, attributed his success in cracking complicated cases to his scrupulous attention to detail.