The arrest of an Ohio couple on drug smuggling charges last week illustrated the investigative methods federal agents are using to track down and build cases against people who sell deadly drugs like fentanyl on the dark web. In this case, a Homeland Security agent, a postal inspector and a drug-sniffing DEA dog named Sadie teamed up to gather evidence against James Halpin and Grace Bosworth, according to the new criminal complaint charging the Cincinnati couple.

The complaint lays out how federal agents are working together across agencies: ordering fentanyl from dark web marketplaces, picking up the drugs from undercover addresses and comparing mailing labels and handwriting samples. The effort comes as fentanyl, about 50 times stronger than heroin, is spreading quickly across the country. 

A task force made up of multiple federal law-enforcement agencies was assigned in May to investigate fentanyl sellers operating within the U.S. by making undercover purchases on dark web marketplaces. That same month, agents from Homeland Security Investigations and the U.S. Postal Inspection Service found a dark website selling fentanyl with offers like, “100mg of Fentanyl HCL 98% purity $105+35 for Express-1 days shipping.”

The agents ordered fentanyl from the website on May 30, directing that it be delivered to an undercover name and address in northern Ohio. The U.S. Express Mail parcel they received carried a plastic baggie of fentanyl concealed inside a magazine and bore a return address from an A.P. Schweitzer in Newport, Kentucky.

A postal inspector in Cincinnati examined the 11-digit label sequence on the “Schweitzer” parcel and found four other packages sent from a post office in Norwood, Ohio—a 15-minute drive from Newport, Kentucky—that bore the same return name and address and were each sent to a different state, Newsweek.com reports.