SHANGHAI: For a few thousand dollars, Chinese companies offer to export a powerful chemical that has been killing unsuspecting drug users and is so lethal that it presents a potential terrorism threat, an Associated Press investigation has found.

The AP identified 12 Chinese businesses that said they would export the chemical — a synthetic opioid known as carfentanil — to the United States, Canada, the United Kingdom, France, Germany, Belgium and Australia for as little as $2,750 a kilogram (2.2 pounds), no questions asked.

Carfentanil burst into view this summer, the latest scourge in an epidemic of opioid abuse that has killed tens of thousands of people in the United States alone. Dealers have been cutting carfentanil and its weaker cousin, fentanyl, into heroin and other illicit drugs to boost profit margins.

Despite the dangers, carfentanil is not a controlled substance in China, where it is manufactured legally and sold openly online. The US government is pressing China to blacklist carfentanil, but Beijing has yet to act, leaving a substance whose lethal qualities have been compared with nerve gas to flow into foreign markets unabated.

"We can supply carfentanil . for sure," a saleswoman from Jilin Tely Import and Export Co. wrote in broken English in a September email. "And it's one of our hot sales product."

China's Ministry of Public Security declined multiple requests for comment from the AP.

Before being discovered by drug dealers, carfentanil and substances like it were viewed as chemical weapons. One of the most powerful opioids in circulation, carfentanil is so deadly that an amount smaller than a poppy seed can kill a person. Fentanyl is up to 50 times stronger than heroin; carfentanil is chemically similar, but 100 times stronger than fentanyl itself.

"It's a weapon," said Andrew Weber, assistant secretary of defense for nuclear, chemical and biological defense programs from 2009 to 2014. "Companies shouldn't be just sending it to anybody."

The AP did not actually order any drugs so could not conduct tests to determine whether the products on offer were genuine. But a kilogram of carfentanil shipped from China was recently seized in Canada.

Carfentanil was first developed in the 1970s, and its only routine use is as an anesthetic for elephants and other large animals. Governments quickly targeted it as a potential chemical weapon. Forms of fentanyl are suspected in at least one known assassination attempt, and were used by Russian forces against Chechen separatists who took hundreds of hostages at a Moscow theater in 2002.

The chemicals are banned from the battlefield under the Chemical Weapons Convention.

In fiscal year 2014, US authorities seized just 3.7 kilograms (8.1 pounds) of fentanyl. This fiscal year, through just mid-July, they have seized 134.1 kilograms (295 pounds), according to US Customs and Border Protection data obtained by the AP. Fentanyl is the most frequently seized synthetic opioid, US Customs reports.

Users are dying of accidental respiratory arrest, and overdose rates have soared. China has not been blind to the key role its chemists play in the global opioid supply chain. Most synthetic drugs that end up in the United In States come from China, either directly or by way of Mexico, according to the DEA. China already has placed controls on 19 fentanyl-related compounds. Adding carfentanil to that list is likely to only diminish, not eliminate, global supply.

Despite periodic crackdowns, people willing to skirt the law are easy to find in China's vast, freewheeling chemicals industry, made up of an estimated 160,000 companies operating legally and illegally. Vendors said they lie on customs forms, guaranteed delivery to countries where carfentanil is banned and volunteered strategic advice on sneaking packages past law enforcement.

Speaking from a bright booth at a chemicals industry conference in Shanghai last month, Xu Liqun said her company, Hangzhou Reward Technology, could produce carfentanil to order.

"It's dangerous, dangerous, but if we send 1kg, 2kg, it's OK," she said, adding that she wouldn't do the synthesis herself because she's pregnant. She said she knows carfentanil can kill and believes it should be a controlled substance in China.

"The government should impose very serious limits, but in reality in China it's so difficult to control because if I produce one or two kilograms, how will anyone know?" she said. "They cannot control you, so many products, so many labs."

Several vendors recommended sending the drugs via EMS, the express mail service of state-owned China Postal Express & Logistics Co., as a fail-safe option.

"EMS is a little slow than Fedex or DHL but very safe, more than 99% pass rate," a Yuntu Chemical Co. representative wrote in an email. "If send to the USA, each package less than 250g is the best, small and unattractive, we will divide 1kg into 4-5 packages and send every other day or send to different addresses."

EMS declined to comment.

A Yuntu representative hung up the phone when contacted by the AP and did not reply to emails seeking comment. Soon after, the company's website vanished.

Not all of the websites used to sell the drugs are based in China. At least six Chinese companies offering versions of fentanyl, including carfentanil, had IP addresses in the United States, hosted at US commercial web providers, the AP found.

"NOBODY WAS MOVING. THEY PUT THE PEOPLE THERE LIKE DOLLS"

In 2002, Russian Special Forces turned to carfentanil after a three-day standoff with Chechen separatists, who had taken more than 800 people hostage in a Moscow theater. They used an aerosol version of carfentanil, along with the less potent remifentanil, sending it through air vents, according to a paper by British scientists who tested clothing and urine samples from three survivors.

The strategy worked, but more than 120 hostages died from the effects of the chemicals.

Olga Dolotova, an engineer who survived the attack, remembers seeing white plumes descending before she lost consciousness. When she awoke, she found herself on a bus packed with bodies. "It was such a horror just to look at it," she said. "Nobody was moving. They put the people there like dolls."

The theater siege raised concerns about carfentanil as a tool of war or terrorism, and prompted the US to develop strategies to counter its use, according to Weber, the former Defense Department chemical weapons expert.

The US, Russia, China, Israel, the Czech Republic, the United Kingdom and India are among the countries that have assessed carfentanil and related compounds for offensive or defensive applications, according to publicly available documents and academic studies.

"Countries that we are concerned about were interested in using it for offensive purposes," Weber said. "We are also concerned that groups like ISIS could order it commercially."

Weber considered a range of alarming scenarios, including the use of carfentanil to knock out and take troops hostage, or to kill civilians in a closed environment like a train station. He added that it is important to raise awareness about the threat from carfentanil trafficking.

"Shining sunlight on this black market activity should encourage Chinese authorities to shut it down," he said.

Fentanyls also have been described as ideal tools for assassination — lethal and metabolised quickly so they leave little trace.

Agents from Israel's secret intelligence service, Mossad, sprayed a substance believed to be a fentanyl analog into the ear of Hamas leader Khaled Mashaal as he walked down a street in Amman, Jordan, in a botched 1997 assassination attempt.

The US began researching fentanyl as an incapacitating agent in the 1960s and, by the 1980s, government scientists were experimenting with aerosolised carfentanil on primates, according to Neil Davison, the author of "'Non-Lethal' Weapons" who now works at the International Committee of the Red Cross.

The US says it is no longer developing such chemical agents. But two state-owned companies in China have marketed "narcosis" dart guns, according to Michael Crowley, project coordinator at the University of Bradford's Non-Lethal Weapons Research Project. He said the ammunition "might very well be fentanyl or an analog of fentanyl," adding that in the 1990s, the US explored similar guns loaded with a form of fentanyl.

Among the problems with fentanyls is that the line between life and death is too thin.

"There is no incapacitating chemical agent that can be used in a tactical situation without extreme risk of injury or death to everybody in the room," Crowley said.

"HURRY TO BUY"

DEA officials say they are getting unprecedented cooperation from China in the fight against fentanyls, noting unusually deep information sharing in what can be a fractious bilateral relationship.

The DEA has "shared intelligence and scientific data" with Chinese authorities about controlling carfentanil, according to Russell Baer, a DEA special agent in Washington.

"I know China is looking at it very closely," he said. "That's been the subject of discussion in some of these high-level meetings."

Last October, China added 116 synthetic drugs to its controlled substances list, which had a profound impact on global narcotics supply chains. Acetylfentanyl, for example, is a weaker cousin of carfentanil that China included on last year's list of restricted substances. Six months later, monthly seizures of acetylfentanyl in the US had plummeted by 60 percent, DEA data obtained by the AP shows.

Several vendors contacted in September were willing to export carfentanil, but refused to ship the far less potent acetylfentanyl. A Jilin Tely Import & Export Co. saleswoman offered carfentanil for $3,800 a kilogram, but wrote, with an apologetic happy face, that she couldn't ship acetylfentanyl because it "is regulated by the government now."

Contacted by the AP, the company said it had never shipped carfentanil to North America and had offered to sell it just "to attract the customer."

Seven companies offered to sell acetylfentanyl despite the ban, however. Five offered fentanyl and two offered alpha-PVP, commonly known as flakka, which also are controlled substances in China.

Liu Feng, deputy general manager of Zhejiang Haiqiang Chemical Co., said his company sent a large order of carfentanil to India last year and has sold smaller amounts to trading companies in Shanghai. He said they also had put false labels on packages for customers.

"Everyone in the industry knows it," he said. "But we just do not say it."

Another company went out of its way to recommend acetylfentanyl. "Our customer feedback that the effect is also very good," Wonder Synthesis emailed in broken English. The company says it has warehouses in the US, Europe, Russia and India.

Wonder Synthesis did not respond to emails seeking comment and the phone number provided did not work. The AP visited its published address in Beijing and found a beauty parlor.

The problem with carfentanil is not limited to the United States. In late June, Canadian authorities seized a kilogram of carfentanil shipped from China in a box labelled printer accessories.

The powder contained 50 million lethal doses, according to the Canada Border Services Agency — more than enough to wipe out the entire population of the country. It was hidden inside bright blue cartridges labelled as ink for HP LaserJet printers. "Keep out of reach of children," read the labels, in Chinese.

Royal Canadian Mounted Police officers in Vancouver sealed themselves inside hazmat suits, binding their wrists, ankles, zippers, and face masks with fat yellow tape. With large oxygen containers on their backs and chunky respirators, it looked as if they were preparing for a trip to the moon.

"Cocaine or heroin, we know what the purpose is," said Allan Lai, an officer-in-charge at the Royal Canadian Mounted Police in Calgary, who is helping oversee the criminal investigation. "With respect to carfentanil, we don't know why a substance of that potency is coming into our country."

In August and September, high-level delegations of Chinese and US drug enforcement authorities met to discuss joint efforts on synthetic opioids, but neither meeting produced any substantive announcement on carfentanil.

Nonetheless, some Chinese vendors are already bracing for a new wave of controls. In an email, Wonder Synthesis wrote, "If you need any chems, just hurry to buy."