[Enrivaanonock]

Meanwhile, patients kept coming, and hospital personnel could barely keep up.

“I ended up giving C.P.R.,” Dr. Sosa said. “I haven’t given C.P.R. since I was a resident, but there were so many crises going on.”

Frightened hospital patients had to watch others around them die for reasons no one understood, fearing that they might be next.

As reports of strange Guillain-Barré symptoms started coming in from other parts of the country, doctors realized they were not just dealing with a localized outbreak.

Pascuala Pérez de González, 67, sought treatment for a cold at a clinic in Coclé Province, about a three-hour drive from Panama City. In late September she was treated and sent home. Within days, she could no longer eat; she stopped urinating and went into convulsions.

A decision was made to transport her to the public hospital in Panama City, but on the way she stopped breathing and had to be resuscitated. She arrived at the hospital in a deep coma and later died.

Medical records contained clues but also plenty of false leads. Early victims tended to be males older than 60 and diabetic with high blood pressure. About half had been given Lisinopril, a blood pressure medicine distributed by the public health system.

But many who did not receive Lisinopril still got sick. On the chance that those patients might have forgotten that they had taken the drug, doctors pulled Lisinopril from pharmacy shelves — only to return it after tests found nothing wrong.

Investigators would later discover that Lisinopril did play an important, if indirect role in the epidemic, but not in the way they had imagined.

A Major Clue

One patient of particular interest to Dr. Sosa came into the hospital with a heart attack, but no Guillain-Barré-type symptoms. While undergoing treatment, the patient received several drugs, including Lisinopril. After a while, he began to exhibit the same neurological distress that was the hallmark of the mystery illness.

“This patient is a major clue,” Dr. Sosa recalled saying. “This is not something environmental, this is not a folk medicine that’s been taken by the patients at home. This patient developed the disease in the hospital, in front of us.”

Soon after, another patient told Dr. Sosa that he, too, developed symptoms after taking Lisinopril, but because the medicine made him cough, he also took cough syrup — the same syrup, it turned out, that had been given to the heart patient.

“I said this has got to be it,” Dr. Sosa recalled. “We need to investigate this cough syrup.”

The cough medicine had not initially aroused much suspicion because many victims did not remember taking it. “Twenty-five percent of those people affected denied that they had taken cough syrup, because it’s a nonevent in their lives,” Dr. Motta said.

Investigators from the United States Centers for Disease Control and Prevention, who were in Panama helping out, quickly put the bottles on a government jet and flew them to the United States for testing. The next day, Oct. 11, as Panamanian health officials were attending a news conference, a Blackberry in the room went off.

The tests, the C.D.C. was reporting, had turned up diethylene glycol in the cough syrup.

The mystery had been solved. The barrels labeled glycerin turned out to contain poison.

Dr. Sosa’s exhilaration at learning the cause did not last long. “It’s our medication that is killing these people,” he said he thought. “It’s not a virus, it’s not something that they got outside, but it was something we actually manufactured.”

A nationwide campaign was quickly begun to stop people from using the cough syrup. Neighborhoods were searched, but thousands of bottles either had been discarded or could not be found.

As the search wound down, two major tasks remained: count the dead and assign blame. Neither has been easy.

A precise accounting is all but impossible because, medical authorities say, victims were buried before the cause was known, and poor patients might not have seen doctors.

Another problem is that finding traces of diethylene glycol in decomposing bodies is difficult at best, medical experts say. Nonetheless, an Argentine pathologist who has studied diethylene glycol poisonings helped develop a test for the poison in exhumed bodies. Seven of the first nine bodies tested showed traces of the poison, Panamanian authorities said.

With the rainy season returning, though, the exhumations are about to end. Dr. José Vicente Pachar, director of Panama’s Institute of Legal Medicine and Forensic Sciences, said that as a scientist he would like a final count of the dead. But he added, “I should accept the reality that in the case of Panama we are not going to know the exact number.”

Local prosecutors have made some arrests and are investigating others connected to the case, including officials of the import company and the government agency that mixed and distributed the cold medicine. “Our responsibilities are to establish or discover the truth,” said Dimas Guevara, the homicide investigator guiding the inquiry.

But prosecutors have yet to charge anyone with actually making the counterfeit glycerin. And if the Panama investigation unfolds as other inquiries have, it is highly unlikely that they ever will.

A Suspect Factory

Panamanians wanting to see where their toxic nightmare began could look up the Web site of the company in Hengxiang, China, that investigators in four countries have identified as having made the syrup — the Taixing Glycerine Factory. There, under the words “About Us,” they would see a picture of a modern white building nearly a dozen stories tall, adorned by three arches at the entrance. The factory, the Web site boasts, “can strictly obey the contract and keep its word.”

But like the factory’s syrup, all is not as it seems.

There are no tall buildings in Hengxiang, a country town with one main road. The Glycerine Factory is not certified to sell any medical ingredients, Chinese officials say. And it looks nothing like the picture on the Internet. In reality, its chemicals are mixed in a plain, one-story brick building.

The factory is in a walled compound, surrounded by small shops and farms. In the spring, nearby fields of rape paint the countryside yellow. Near the front gate, a sign over the road warns, “Beware of counterfeits.” But it was posted by a nearby noodle machine factory that appears to be worried about competition.

The Glycerine Factory bought its diethylene glycol from the same manufacturer as Mr. Wang, the former tailor, the government investigator said. From this spot in China’s chemical country, the 46 barrels of toxic syrup began their journey, passing from company to company, port to port and country to country, apparently without anyone testing their contents.

Traders should be thoroughly familiar with their suppliers, United States health officials say. “One simply does not assume that what is labeled is indeed what it is,” said Dr. Murray Lumpkin, deputy commissioner for international and special programs for the Food and Drug Administration.

In the Panama case, names of suppliers were removed from shipping documents as they passed from one entity to the next, according to records and investigators. That is a practice some traders use to prevent customers from bypassing them on future purchases, but it also hides the provenance of the product.

The first distributor was the Beijing trading company, CNSC Fortune Way, a unit of a state-owned business that began by supplying goods and services to Chinese personnel and business officials overseas.

As China’s market reach expanded, Fortune Way focused its business on pharmaceutical ingredients, and in 2003, it brokered the sale of the suspect syrup made by the Taixing Glycerine Factory. The manufacturer’s certificate of analysis showed the batch to be 99.5 percent pure.

Whether the Glycerine Factory actually performed the test has not been publicly disclosed.

Original certificates of analysis should be passed on to each new buyer, said Kevin J. McGlue, a board member of the International Pharmaceutical Excipients Council. In this case, that was not done.

Fortune Way translated the certificate into English, putting its name — not the Glycerine Factory’s — at the top of the document, before shipping the barrels to a second trading company, this one in Barcelona.

Li Can, managing director at Fortune Way, said he did not remember the transaction and could not comment, adding, “There is a high volume of trade.”

Upon receiving the barrels in September 2003, the Spanish company, Rasfer International, did not test the contents, either. It copied the chemical analysis provided by Fortune Way, then put its logo on it. Ascensión Criado, Rasfer’s manager, said in an e-mail response to written questions that when Fortune Way shipped the syrup, it did not say who made it.

Several weeks later, Rasfer shipped the drums to a Panamanian broker, the Medicom Business Group. “Medicom never asked us for the name of the manufacturer,” Ms. Criado said.

A lawyer for Medicom, Valentín Jaén, said his client was a victim, too. “They were tricked by somebody,” Mr. Jaén said. “They operated in good faith.”

In Panama, the barrels sat unused for more than two years, and officials said Medicom improperly changed the expiration date on the syrup.

During that time, the company never tested the product. And the Panamanian government, which bought the 46 barrels and used them to make cold medicine, also failed to detect the poison, officials said.

The toxic pipeline ultimately emptied into the bloodstream of people like Ernesto Osorio, a former high school teacher in Panama City. He spent two months in the hospital after ingesting poison cough syrup last September.

Just before Christmas, after a kidney dialysis treatment, Mr. Osorio stood outside the city’s big public hospital in a tear-splattered shirt, describing what his life had become.

“I’m not an eighth of what I used to be,” Mr. Osorio said, his partly paralyzed face hanging like a slab of meat. “I have trouble walking. Look at my face, look at my tears.” The tears, he said apologetically, were not from emotion, but from nerve damage.

And yet, Mr. Osorio knows he is one of the lucky victims.

“They didn’t know how to keep the killer out of the medicine,” he said simply.

While the suffering in Panama was great, the potential profit — at least for the Spanish trading company, Rasfer — was surprisingly small. For the 46 barrels of glycerin, Rasfer paid Fortune Way $9,900, then sold them to Medicom for $11,322, according to records.

Chinese authorities have not disclosed how much Fortune Way and the Taixing Glycerine Factory made on their end, or how much they knew about what was in the barrels.

“The fault has to be traced back to areas of production,” said Dr. Motta, the cardiologist in Panama who helped uncover the source of the epidemic. “This was my plea — please, this thing is happening to us, make sure whoever did this down the line is not doing it to Peru or Sierra Leone or some other place.”

A Counterfeiter’s Confession

The power to prosecute the counterfeiters is now in the hands of the Chinese.

Last spring, the government moved quickly against Mr. Wang, the former tailor who poisoned Chinese residents.

The authorities caught up with him at a roadblock in Taizhou, a city just north of Taixing, in chemical country. He was weak and sick, and he had not eaten in two days. Inside his white sedan was a bankbook and cash. He had fled without his wife and teenage son.

Chinese patients were dead, a political scandal was brewing and the authorities wanted answers. Mr. Wang was taken to a hospital. Then, in long sessions with investigators, he gave them what they wanted, explaining his scheme, how he tested industrial syrup by drinking it, how he decided to use diethylene glycol and how he conned pharmaceutical companies into buying his syrup, according to a government official who was present for his interrogation.

“He made a fortune, but none of it went to his family,” said Wang Xiaodong, a former village official who knows Mr. Wang and his siblings. “He liked to gamble.”

Mr. Wang remains in custody as the authorities decide whether he should be put to death. The Qiqihar drug plant that made the poisonous medicine has been closed, and five employees are now being prosecuted for causing “a serious accident.”

In contrast to the Wang Guiping investigation, Chinese authorities have been tentative in acknowledging China’s link to the Panama tragedy, which involved a state-owned trading company. No one in China has been charged with committing the fraud that ended up killing so many in Panama.

Sun Jing, the pharmaceutical program officer for the World Health Organization in Beijing, said the health agency sent a fax “to remind the Chinese government that China should not be selling poisonous products overseas.” Ms. Sun said the agency did not receive an official reply.

Last fall, at the request of the United States — Panama has no diplomatic relations with China — the State Food and Drug Administration of China investigated the Glycerine Factory and Fortune Way.

The agency tested one batch of glycerin from the factory, and found no glycerin, only diethylene glycol and two other substances, a drug official said.

Since then, the Chinese drug administration has concluded that it has no jurisdiction in the case because the factory is not certified to make medicine.

The agency reached a similar conclusion about Fortune Way, saying that as an exporter it was not engaged in the pharmaceutical business.

“We did not find any evidence that either of these companies had broken the law,” said Yan Jiangying, a spokeswoman for the drug administration. “So a criminal investigation was never opened.”

A drug official said the investigation was subsequently handed off to an agency that tests and certifies commercial products — the General Administration of Quality Supervision, Inspection and Quarantine.

But the agency acted surprised to learn that it was now in charge. “What investigation?” asked Wang Jian, director of its Taixing branch. “I’m not aware of any investigation involving a glycerin factory.”

Besides, Huang Tong, an investigator in that office, said, “We rarely get involved in products that are sold for export.”

Wan Qigang, the legal representative for the Taixing Glycerine Factory, said in an interview late last year that the authorities had not questioned him about the Panama poisoning, and that his company made only industrial-grade glycerin.

“I can tell you for certain that we have no connection with Panama or Spain,” Mr. Wan said.

But in recent months, the Glycerine Factory has advertised 99.5 percent pure glycerin on the Internet.

Mr. Wan recently declined to answer any more questions. “If you come here as a guest, I will welcome you,” Mr. Wan said. “But if you come again wanting to talk about this matter, I will make a telephone call.”

A local government official said that Mr. Wan was told not to grant interviews.

A five-minute walk away, another manufacturer, the Taixing White Oil Factory, also advertises medical glycerin on the Internet, yet it, too, has no authorization to make it. The company’s Web site says its products “have been exported to America, Australia and Italy.”

Ding Xiang, who represents the White Oil Factory, denied that his company made pharmaceutical-grade glycerin, but he said chemical trading companies in Beijing often called, asking for it.

“They want us to mark the barrels glycerin,” Mr. Ding said in late December. “I tell them we cannot do that.”

Mr. Ding said he stopped answering calls from Beijing. “If this stuff is taken overseas and improperly used. ...” He did not complete the thought.

In chemical country, product names are not always what they seem.

“The only two factories in Taixing that make glycerin don’t even make glycerin,” said Jiang Peng, who oversees inspections and investigations in the Taixing branch of the State Food and Drug Administration. “It is a different product.”

All in a Name

One lingering mystery involves the name of the product made by the Taixing Glycerine Factory. The factory had called its syrup “TD” glycerin. The letters TD were in virtually all the shipping documents. What did TD mean?

Spanish medical authorities concluded that it stood for a manufacturing process. Chinese inspectors thought it was the manufacturer’s secret formula.

But Yuan Kailin, a former salesman for the Glycerine Factory, said he knew what the TD meant because a friend and former manager of the factory, Ding Yuming, had once told him. TD stood for the Chinese word “tidai” (pronounced tee-die), said Mr. Yuan, who left his job in 1998 and still lives about a mile from the factory.

In Chinese, tidai means substitute. A clue that might have revealed the poison, the counterfeit product, was hiding in plain sight.

It was in the product name.

Renwick McLean and Brent McDonald contributed reporting.