In this Sept. 21, 2016 photo, a worker looks out from the Shanghai Xianchong Chemical Co. in Shanghai, China. The company offered 4-fluoroisobutyrylfentanyl, 4-FIBF for short, for sale and operates from a small office in central Shanghai. They started fielding requests for 4-FIBF around April 2016, according to the manager named Jammi Gao. The proliferation of rapidly evolving synthetic opioids has become so fierce that the Drug Enforcement Administration says they now constitute an entire new class of drugs, which are fueling the deadliest addiction crisis the United States has ever seen. The fentanyl-like drugs are pouring in primarily from China, U.S. officials say though Beijing maintains that assertion is unsubstantiated. (AP Photo/Andy Wong)
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