While bank accounts, and call or chat logs can be used in court, they can only be obtained illegally－by, say, conspiring with bank employees, public security bureaus or communications companies.

"Evidence gathered illegally can't be used in court," Wang Xiuquan points out. "But lawyers can apply to the court to collect illegally procured evidence, making it legally presentable."

Yet lawyers' ability to do so is limited, and the procedures are complicated, he says. Private investigators could play a greater role if lawyers have more rights.

Renmin University law professor He Jiahong says: "Some people use illegal methods to gather information. Their actions are no different from gangsters'."

Wang, the lawyer, says a PI once attempted to blackmail his client, a company boss, after discovering the man was having an affair.

The industry should be regulated by clarifying its legal classification and placing it under such frameworks as the Forensic Investigation Law, He says.

"It'd be good if private investigators' selection and training could be like lawyers' judicial examinations," he believes.

Instead, they exist in legal limbo. The Ministry of Public Security issued a 1993 directive against private investigation, pointing out PIs sometimes use powers exclusive to law enforcement. "But it's just a notice－not a law," He says.

"Economic and legal reform have increased awareness about evidence's role in the judicial system. Government departments can't keep up with the proliferation of extramarital affairs, business disputes and IPR violations. Private detectives are meeting demand."

Wang Dacheng says: "We lack legal status. But growing demand shows we have value. I don't blame anyone, but the reality is that law enforcement sometimes can't help the helpless, while private investigators can."

The industry can help the government maintain social order and reduce costs if properly managed, He believes. He prefers the term "folk investigators" because it doesn't connote privacy violations.

Mu says: "Private investigation can complement the justice system to help public security departments solve cases," he says. "If the government can guide and regulate the industry, I'm sure more cases, including criminal cases, can be solved. The police could take more holidays."

He points out the industry is better controlled than a decade before. "But we're still like underground guerillas. We work without a sense of achievement," Mu says.

"I really hope one day I can publicly identify myself as a legal private detective."

Han Junhong contributed to this story.