“A simple Google search of ‘fentanyl for sale’ returned a number of potential sellers,” according to a Senate Homeland Security Committee report released in January.

It said investigators, “posing as a first-time fentanyl purchaser,” had contacted six online sellers overseas, and each offered to ship purchases to the United States — sometimes with aggressive salesmanship.

The sellers “actively negotiated … to complete a deal by offering flash sales on certain illicit opioids and discounted prices for bulk purchases,” the report said. When investigators “failed to immediately respond to an offer, the online sellers proactively followed up, sometimes offering deeper discounts to entice a sale.”

Fentanyl was developed decades ago as an ultra-powerful painkiller — 100 times more potent than morphine — for use in surgery. It is still used to help hospice-level cancer patients.

Drug dealers began dabbling in the drug in the mid-2000s, but it surged in popularity in 2014 and 2015 because it was easy to obtain and hugely profitable.

A $1,500 kilogram can bring $1.5 million in profits after the drug is cut and sold on the street, according to the Drug Enforcement Administration.

There was only one place to obtain the drug: China. It has a robust chemical and pharmaceutical sector, as well as lax regulations and widespread corruption.

“Regulatory gaps have led to a large increase in the number of unlicensed or ‘semi-legitimate’ chemical manufacturers or distributors,” Bryce Pardo, an analyst from the Rand Corp. think tank, recently told Congress.

“A lack of oversight and government and corporate accountability increase opportunities for corruption,” he added.

Chinese dealers targeted a loophole that let them send packages to the United States 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 such information to customs inspectors, which can help authorities identify smugglers and smuggling patterns.

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

U.S. officials long have pressed China to more aggressively police its chemical manufacturers, and China has strictly regulated the production of 175 chemicals, including fentanyl and some of its analogs.

That chemical-by-chemical approach, however, permits drug companies to tweak chemical formulas to get around a ban.

U.S. officials want China to follow the lead of the DEA, which in February used emergency powers to categorize fentanyl-related substances as controlled substances under federal law. The move was designed to make it easier to prosecute offenders and thwart chemists from slightly altering formulas.

A Chinese Embassy representative in Washington declined comment but forwarded remarks made by Hua Chunying, a Ministry of Foreign Affairs official, at a January news conference in Beijing.

“Anti-drug cooperation is one of the highlights of China-U.S. law enforcement cooperation,” Hua said. “In recent years, the two sides have conducted some highly effective cooperation on cracking down on cross-border drug-related crimes and advancing psychoactive substance listing and control, which has won wide approval from the public of the two sides. China’s attitude on this issue is very clear.”

The Justice Department has brought charges against several Chinese manufacturers of synthetic opioids. They are unlikely to end up in U.S. courtrooms because Washington and Beijing do not have an extradition treaty, and China has generally refused to send its citizens to the United States for criminal trials.

Last October, the Justice Department unveiled the first-ever indictment against Chinese manufacturers of opioids, accusing two groups with operating illicit labs that sold fentanyl and other drugs to U.S. dealers. The rings were vast — one involved at least 100 distributors — and authorities were able to trace at least four deaths to fentanyl and related chemicals sold by one of the groups, court records show.

Then in August, federal prosecutors in Cleveland unveiled a 43-count indictment against the Zheng organization. It alleged that Fujing Zheng, 35, and his father, Guanghua Zheng, 62, both of Shanghai, ran a global organization that manufactured tons of illicit chemicals each month.

U.S. officials said the Zhengs were adept at staying ahead of regulators — and police. When China banned unregulated production of one synthetic narcotic, officials said, the Zhengs used their expertise to adjust the formula to skirt the prohibitions and keep the drugs flowing.

“We work diligently to make every possible chemical to meet the needs of our customers,” the Zhengs wrote on one of their websites, according to court papers. “We will create custom-made products for you.”

To get their product to U.S. customers, the Zhengs often relied on middlemen who hid the drugs in bulk freight shipments and then helped redistribute them. Prosecutors said that helped obscure the narcotics’ origins.

The Zhengs could not be reached for comment for this story.

But their operation had a deadly effect halfway across the world. In February 2015, Leroy Steele, 38, a small-time drug dealer in the Akron, Ohio, area, sent the Zheng organization an email seeking to purchase acetyl fentanyl, according to court papers.

“Send me prices as well as information on where I can send the money,” Steele wrote.

A member of the Zheng group quickly replied, prosecutors alleged, and claimed he represented “professional acetyl fentanyl manufacturer in China, our products are all best quality, a lot of U.S. and Europe customers purchase largely from us every month.”

“Tell me how many quantity you wanna buy,” the member wrote. “Do you wanna have a sample order?”

Steele wired the Zhengs $3,500 for half a kilogram of the narcotic.

The dealer, who later would be sentenced to 20 years in prison after pleading guilty to drug distribution charges, soon was slinging heroin laced with acetyl fentanyl to his customers, prosecutors said.

Within a few weeks, federal officials said, two of Steele’s customers, a 37-year-old man and a 23-year-old woman, were dead — from overdoses.