CHANGCHUN, JUNE 11 -- Yang Rui did not hear from his son Yang Jiemin for six months after he graduated from college.

Desperate to find Jiemin, the father turned to Li Yuan, who runs an independent investigative firm in the northeast Chinese city Changchun, capital of Jilin Province.

Li tracked Jiemin down based on a phone number he once used to call Yang to ask for money. Jiemin was being held against his will by a gang plotting pyramid schemes. Li eventually pinned down his exact location with the help of a delivery man who took food to the gang. Li reported the matter to local police who found Jiemin and busted the crooks involved.

In the three years Li has worked as a private investigator, most of his clients have been seeking lost parents or runaway kids. Their cases usually do not meet the minimum criteria for police to launch a formal investigation. Others have asked them to find out if their spouses are cheating on them and want evidence.

A growing legion of private investigators, "detectives" as some call themselves, are meeting a wide range of needs from individuals and companies that are not addressed through conventional legal channels.

Services range from snooping on extramarital affairs to corporate profiling and debt recovery. Many advertise unique strengths and all pledge to keep their clients' information confidential.

Chen Tianben of the People's Public Security University of China has studied private investigators in China. He estimates that more than 100,000 people are employed in nearly 23,000 firms. Many of these investigators are former employees of the judiciary, law enforcers or lawyers.

Li's agency charged Yang 30,000 yuan (around 4,800 U.S. dollars) to find his son. The agency asked the couple to pay a retainer of 5,000 yuan when they took the case and the rest when the case was closed. If the agency had failed to find Jiemin, Li said the company would charge only 7,000 yuan.

Many private investigators contacted by Xinhua asked their clients to deposit around 50-60 percent of their estimated fee in advance. Some ask clients to pay when they make major breakthroughs.

Private investigators first emerged in China in the early 1990s in Beijing and Shanghai, usually retained to assist civil, and occasionally criminal, investigations.

Some agencies recover debts through threats and extortion, forcing police into a difficult position. Many of the methods they use are not legal and may constitute criminal offenses. Some of the business they engage in also falls into a gray area in law. In 1993, private investigative agencies were outlawed.

Peter William Humphrey, 58, a British national and his wife, Chinese-born American Yu Yingzeng, 61, were sentenced to two years in prison and fined 150,000 yuan last year for illegally obtaining private information on Chinese citizens for GlaxoSmithKline.

Such cases highlight the legal quandaries facing investigators. A lack of official recognition and clear legal guidance means upright and law-abiding investigators are often tarred with the same brush as those engaged in illegal undertakings, Chen said. No professional code of conduct regulates their practices. Professionalism and legal compliance are patchy. No legislation clarifies what is off-limits.

"Some so-called investigators have violated citizens' right to privacy, and many justify their actions as due diligence or saving a marriage." said a Changchun police officer.

Leading firms are very strict in compliance with local laws and regulations. Investigators can only track quarry in public places and are only allowed to collect information from public sources. Many firms forbid investigators from using wiretaps or spyware.

Due diligence investigations and probes into corruption are among services requested by multinational corporations operating in China. Paradoxically, it appears to be the Chinese authorities themselves, through their intense fight against corruption, who have inflamed foreign business concerns.

Despite being unrecognized in China, private investigators have a role to play in litigation, both civil and criminal, as lawyers sometimes hire them to collect evidence. China's Civil Procedure Law stipulates that the burden of proof falls on the claimant. Lawyers say many claimants who cannot retrieve effective evidence on their own naturally seek help from "professional services."

"The existence of so many investigative firms clearly points to a public need for such services," said Wang Zhiyuan of Jilin University. "The law should be amended to regulate their business and uphold professionalism and ethics."