Who's responsible for the fentanyl crisis?

Image | Fentanyl Trafficking 20150811

Caption: Fentanyl pills are shown in an undated police handout photo. (The Canadian Press)

The U.S is cracking down on fentanyl's global supply chain by targeting Mexican and Chinese individuals with indictments and sanctions. Officials from those countries insist it's largely an American problem.
What's actually happening on the ground in China and Mexico? What impact will U.S. retaliation have?
Today, journalist Zachary Siegel explains.
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