A Cincinnati-area couple used the Postal Service for next-day delivery of one of the most dangerous drugs on the market, federal officials said.

James Halpin, 30, and Grace Bosworth, 38, stand accused of smuggling, importing and distributing fentanyl, a deadly synthetic opiate, and are awaiting court appearances, federal agents said Monday.

They would charge $35 for next-day shipping, court documents show. Undercover officers say they ordered drugs off of the murkiest part of the Internet called the dark Web, receiving them the next day between the pages of a magazine.

A key to cracking the case: Authorities compared the handwriting on a shipping label to that found on Bosworth’s divorce papers.

In late May, with the approval from the U.S. Attorney’s Office in Cleveland, a number of undercover law enforcement agencies started making purchases from fentanyl vendors on the dark Web, a network of websites searchable only using identity-masking software. As a result, sites on the dark Web can't be found through search engines.

Several high-profile cases involving child pornography, drugs, weapons and human trafficking have all been tied to dark Web activity.

Authorities say Halpin and Bosworth operated an online storefront on the dark Web as a way for people to order fentanyl, which then would be shipped using the U.S. Postal Service. They are also accused of "routinely" receiving shipments of drugs from Canada.

The Postal Service serves as a primary conduit for such drugs coming into the United States, mostly from China, according to law-enforcement officials.

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Halpin received a lot of international mail at his address, and he routinely sent mail to people all over the country, inspectors learned. Bosworth, who lives with Halpin in a house she owns, was traced to the packages through her handwriting, documents said.

On June 6, when a package from Montreal, arrived at a Norwood post office with the couple's address, the contents were sent to the Hamilton County Coroner’s Office for testing.

The package was found to contain 5 grams of a mixture of fentanyl, carfentanil and other fentanyl analogs, according to court documents. Five grams is about one teaspoon.

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When Halpin came to the post office the next day to pick up the package and ship out others, authorities arrested him. At the time of his arrest, Halpin was found with “untested fentanyl” in a tobacco snuff can, documents said.

Halpin admitted to police that he had shipped the packages but said Bosworth was the one who had packaged, bought and sold the drugs, according to agents. Halpin told investigators they had been receiving the packages of drugs from Canada weekly since March, and that each contained at least 2 grams of fentanyl.

He also said Bosworth asked him to buy bitcoins, an online digital currency, but initially didn't know it was for drugs.

The federal investigators found mailing receipts, drugs and items to package and ship drugs at two locations, court documents state.

Agents found Bosworth at one of those places. In her purse they discovered a substance believed to be fentanyl, the court documents state.

Bosworth is listed as the founder of Global 2 Local Language Solutions in Norwood, and is listed as its business agent, according to filings with the Ohio Secretary of State.

She is also the registered agent for five companies with names similar to Sunny Day Real Estate, all which operate out of the same Norwood address. On LinkedIn, a Jim Halpin is listed as chief operating officer of Local Language Solutions.

Contributing: Terry DeMio, The Cincinnati Enquirer. Follow Sarah Brookbank on Twitter: @SarahBrookbank