But to determine the origin of the steroids, investigators had to look to only one place: China.

Since at least 2005, when a US-led crackdown crippled Mexico's steroid industry, Chinese firms have been supplying the vast majority of the steroids sold over the internet, according to US law enforcement officials.

Those steroids are used by a wide range of athletes, from amateur bodybuilders to top-tier professionals.

Now, because of US pressure and with the Olympics Games in Beijing only eight months away, mainland authorities say they are scrutinizing the companies accused of exporting steroids and vowing to better police the trade. But interviews with steroid manufacturers, suppliers and government authorities highlighted the barriers to effectively containing the problem: confusion over regulations reigns, and companies can manipulate the system.

American investigators have identified at least 37 Chinese companies that are involved in the illicit trade of steroids. Many of the companies could not be reached. Of the firms that were contacted, 10 said they no longer sold steroids or no longer exported them. But four said they could sell the compounds. One company appeared to have shut its operations in one province only to reopen in another.

Several companies expressed bafflement at the sudden scrutiny by the Chinese government, saying vague laws made unclear exactly what was illegal.

The steroids issue is particularly sensitive as China prepares to host its first Olympics. Mainland authorities are determined to use the Games as an opportunity to showcase their country as a modern state and are keen to prevent any potential embarrassments.

China's State Food and Drug Administration last month announced a five-agency investigation and a plan to "standardize" production and distribution of steroids.

While the country's FDA has provided no further details and would not comment specifically on the mainland companies being targeted, it has said the investigation is the first of its size and scope.

International law enforcement officials with knowledge of the earlier US investigation are hopeful that the Chinese can curb the trade but also skeptical, and they note that they have not received much communication from Beijing.

"The biggest question is sustainability. Is this going to last past the Olympics?" said one official, who spoke on condition of anonymity. "We need to see some action. They need to prove themselves. We're seeing inklings of progress, but in other respects we're seeing nothing."

There are some signs that the mainland investigation is having an effect, at least for now. In interviews, several Chinese steroid companies reported having been questioned by authorities and having received orders to halt exports.

Meanwhile, at a pharmaceutical industry conference in Shenzhen last month, several companies known to have sold steroids were noticeably absent, attendees said. Other companies complained that the investigation had hurt their business and said that they had turned to selling other products.

A recent visit to the town of Xianju, in coastal Zhejiang province, where many chemical factories are based, showed how factory workers and law enforcement officials remain confused about exactly what they can produce and export. "Before, the policy was very ambiguous," said a man in the receptionist's office of Zhejiang Xianju Junye Pharmaceutical, who insisted on being identified only by his surname, Li. "But since customs began forbidding the export of steroids, we know we are not allowed."

One firm targeted by US investigators said Chinese companies continued to export illicit drugs to developing countries in Africa and several in South Asia. Other companies said there was no problem selling steroids domestically or to countries other than the United States.

International sport officials say the easy availability of steroids from China has long created a problem for the Olympics. That fact was driven home when court documents revealed that Chinese raw materials had been used in the designer steroids provided by the notorious Bay Area Laboratory Co-Operative, or BALCO, and used by prominent Olympic athletes, including US track-and-field star Marion Jones, at the 2000 Summer Games in Sydney and the 2003 track and field World Championships in Paris.

The outgoing chairman of the World Anti-Doping Agency, Richard Pound, has demanded action on the issue from China for more than a year, saying its reputation as a widespread source for steroids could affect the credibility of the Games this summer, particularly if China fielded a team of extraordinary athletes that seemed to come out of nowhere to win medals.

THE WASHINGTON POST