WASHINGTON -- His moniker was "DARKKING22," and authorities say he offered a cornucopia of illicit drugs through the click of a mouse. But it was his ads on a hidden website for pure fentanyl, the powerful painkiller driving a record number of overdose deaths across the U.S., that caught FBI agents' attention.

They bought some, and days later it arrived in a small, clear, plastic bag complete with a thank-you card -- a sign of how easy it is to buy drugs on the so-called darknet.

The dealer did not stay anonymous for long. Federal authorities say "DARKKING22" was 28-year-old Antoin Austin, of Euclid, Ohio. His arrest last week is among the first by a new team of federal agents, computer experts and analysts tasked with fighting the kind of online opioid trafficking that law enforcement officials say can be more persistent and vexing than more traditional trafficking by cartels.

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Frustrating authorities in their pursuit of online dealers is the anonymity in which they work. Buyers access stores through secret web browsers and make purchases using encrypted channels, code names and virtual currency such as bitcoin.

Attorney General Jeff Sessions said darknet vendors are "pouring fuel on the fire of the national drug epidemic" and this year doubled the number of federal agents working on those cases. It's part of the Trump administration's tough approach to the drug crisis that has focused on harsh punishments for dealers. Critics say the overall strategy resembles a return to failed drug-war tactics and that the record $4.6 billion included in the spending plan the president signed last month is not nearly enough to establish the kind of treatment system needed to reverse the crisis.

But there's bipartisan agreement that more resources and new restrictions are needed to stem the stream of opioids entering the U.S. from overseas, especially China, and into households through the U.S. mail.

Darknet marketplaces are thriving, even after authorities in recent years dismantled two of the most notorious, the Silk Road and AlphaBay, where hundreds of thousands of customers bought not just illegal drugs but weapons, malware and counterfeit and stolen identification. Authorities on the new task force, the Joint Criminal Opioid Darknet Enforcement team, in this case targeted the vendors who sell illicit fentanyl by mail.

"It's not enough simply to take the sites down," Acting Deputy Assistant Attorney General Richard Downing said. "Vendors look for another opportunity, another site, another place where they can go and sell their drugs and guns."

In its first operation, the team arrested Austin and seven others and seized weapons, computer equipment and more than 2,000 lethal doses of the deadly drug. Investigators uncovered 19 overdose deaths they believe are connected to fentanyl purchased online.

Austin's public defender declined to comment.

Before the team's formation, federal agents would dive into complicated investigations largely on their own, sometimes without realizing others were already on the case. But the team has forged a new level of cooperation that its members say is critical in increasingly sophisticated darknet cases that combine tech savvy with old-fashioned drug dealing.

Agents in Pittsburgh, for example, have found drug gangs that traditionally peddled narcotics smuggled from beyond the southern border are now also selling drugs from China online, said Shawn A. Brokos, a supervisory special agent there. Buyers sometimes turn to the dark web for drugs to then deal on the street.

"A lot of them start on a smaller level and then they see how lucrative this can be and they keep expanding," she said.

Investigators from several agencies made a list of "targets," not just the fentanyl peddling vendors, but buyers and users, then set out to contact them, building on existing intelligence. They spoke to more than 160 people, getting a clearer picture of the landscape, said Kyle Rau, of the U.S. Postal Inspection Service.

"They share their secrets, they share their tradecraft," said Emily Odom, chief of the FBI's Hi-Tech Organized Crime Unit. "They're working together, so we have to do a better job working together as well."