The Zheng drug trafficking organization was hardly clandestine. The Shanghai-based network sold synthetic narcotics, including deadly fentanyl, on websites posted in 35 languages, from Arabic and English to Icelandic and Uzbek.

The Chinese syndicate bragged that its laboratory could “synthesize nearly any” drug and that it churned out 16 tons of illicit chemicals a month. The group was so adept at smuggling, and so brazen in its marketing, that it offered a money-back guarantee to buyers if its goods were seized by U.S. or other customs agents.

Over the past decade, federal officials say, the Zheng group mailed and shipped fentanyl and similar illicit chemicals to customers in more than 25 countries and 35 U.S. states. U.S. officials say the syndicate’s success, laid bare in a federal indictment, partly helps explain the United States’ skyrocketing death toll from drug overdoses.

Fentanyl — 50 times more potent than heroin — and related laboratory-crafted drugs have become the No. 1 cause of opioid-related overdose deaths. And rogue chemical companies in China — operating openly and outside the reach of U.S. authorities — are the largest single source of the deadly drugs, law enforcement officials say.

“People in labs in China are producing this substance that is killing Americans,” Deputy Attorney General Rod Rosenstein said. “This is a real crisis.”

U.S. officials have pushed Beijing to shut down the labs and Chinese authorities have taken steps to police chemical makers. The push comes even as relations with Beijing have grown acrimonious amid an escalating trade war and U.S. unease over China’s economic and military clout.

Nearly 29,000 people died last year in the United States from overdoses linked to synthetic opioids, a category that experts say is dominated by fentanyl and its chemical cousins — a surge from the 3,100 such deaths reported in 2013.

One reason for the increase: The drug is so powerful that a sugar-packet-sized bag of it can contain 500 lethal doses. That also means it can be smuggled through the mail in what officials call micro-shipments, which are far harder to identify and interdict than bulkier loads of heroin, cocaine or marijuana.

Chinese companies send fentanyl in small quantities to dealers in the United States or Canada but ship the drugs in bulk to criminal cartels in Mexico. The cartels then mix the synthetics into heroin and other substances or press them into counterfeit pills. The product is then smuggled across the border.

While total fentanyl seizures more than doubled last year, to 1,196 pounds, officials say far more of the illicit drug is getting through. Some of the biggest fentanyl busts have been in California because of the Mexican connection.

In September, for example, U.S. Customs and Border Protection agents seized 52 pounds of powdered fentanyl at the Pine Valley checkpoint near San Diego. In December, officers discovered nearly 80 pounds in a student’s car.

This summer, authorities discovered 20,000 fentanyl pills in a hidden compartment of a Mini Cooper at the San Ysidro checkpoint — a week after confiscating 11,500 pills in another vehicle.

U.S. drug dealers also purchase directly from China on company websites or in so-called dark web drug bazaars, where communications are encrypted and dealers often pay with cryptocurrencies or gift cards that are difficult to trace.

A 33-year-old Long Beach man, for example, was sentenced in June to more than 26 years in federal prison for illegally importing chemicals in bulk from China, including a fentanyl analog, and then producing tens of thousands of pills in a homemade lab. During a nine-month span, prosecutors said, his lab sold an estimated 300,000 pills nationwide.

In Salt Lake City, a former Eagle Scout is awaiting trial after he and five others were charged with turning his mother’s basement into an illicit pill lab. When her house was raided in November 2016, police found 70,000 pills laced with fentanyl and $1.2 million in cash, prosecutors said. The group allegedly sold hundreds of thousands of the pills on the dark web.

The ease with which dealers can buy fentanyl from China “is a challenge because it’s creating traffickers who are not affiliated with larger organizations or with cartels,” said Paul Knierim, a Drug Enforcement Administration official.

Chinese dealers targeted a loophole that let them send packages to the U.S. through the mail without providing detailed information on the sender or the contents of the package. Private carriers such as FedEx and UPS are required to provide that information to customs inspectors.

Congress last month passed legislation designed to close that gap, and President ­Donald Trump is expected to sign it into law.