In China, the top global source of synthetic drugs, carfentanil is not a controlled substance. The U.S. government is pressing China to blacklist it, but Beijing has yet to act.
Carfentanil is 100 times more powerful than fentanyl, a related drug that is itself up to 50 times stronger than heroin.
In fiscal year 2014, U.S. authorities seized just 3.7 kilograms of fentanyl.
Most synthetic drugs that end up in the U.S. come from China, according to the DEA.
China already has controlled fentanyl and 18 related compounds, but despite periodic crackdowns, people willing to skirt the law are easy to find in China's vast, freewheeling chemicals industry.
Last October, China added 116 synthetic drugs to its controlled substances list.
Six months later, monthly seizures of acetylfentanyl in the U.S. were down 60 percent, DEA data obtained by the AP shows.
Five offered fentanyl and two offered alpha-PVP, commonly known as flakka, which are also controlled substances in China.
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