For much of his career as a federal drug agent, Ray Donovan had a singular focus: the capture of Mexican cartel boss Joaquin “El Chapo” Guzman. When Guzman was finally arrested in early 2014, fentanyl trafficking was on the rise in the U.S., and Donovan soon found his next project.

He enlisted a data scientist in an effort to map out the fentanyl networks operating on the East Coast. Reviewing the telephone records of suspected traffickers, investigators saw a pattern of activity that shocked them.

“A ton of calls to China,” recalled Michael Mezner, one of the DEA agents leading the effort. “I looked at that data from every way, and I went to the data guy and told him, ‘I don’t know what’s going on here, but I’ve never seen anything like this in my life.’”

It was a new dynamic: Chinese criminal groups were laundering drug money for the Mexican cartels on an unprecedented scale.