President-elect Donald Trump's plan to combat the opioid crisis largely focuses on drug trafficking from Mexico. But in terms of synthetic opioids such as fentanyl and carfentanil — powerful narcotics that have contributed to mass overdoses across the United States in recent years — the problem appears to originate in China, according to an Associated Press investigation.

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Last week, Chinese officials dismissed claims by U.S. drug enforcement officials that the country was behind the surge in synthetics. In a letter to the Drug Enforcement Administration (DEA), China’s National Narcotics Control Commission said the claims "lack the support of sufficient numbers of actual, confirmed cases."

But according to the DEA, Chinese labs remain the primary manufacturers producing synthetic opioids that are sold online and mailed to the U.S., Canada, or Mexico. DEA spokesperson Melvin Patterson told ATTN: there were two factors driving the distribution channel.

"The main reason there's a lot of synthetics [in China is] that those [drugs] are controlled in the United States," Patterson said. "You're going to face some sort of prosecution if you have those illegally [in the U.S.], and they're not all controlled in China."

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But Patterson added that synthetic trafficking out of China likely wouldn't be an issue were it not for American demand. "Our appetite for those illegal substances fuels the drive in China to supply that," he said. "You get really, really basic into supply and demand. If we didn't have the demand, they wouldn't supply that to us."

There has been progress on the synthetic opioid front. In 2015, the DEA worked with Chinese officials to enact bans on 115 chemicals — including several other synthetic drugs such as K2, Spice, and Flakka — and since then, law enforcement agencies have seen dramatic declines in the use and trafficking of these substances, CNN reported. But the problem persists because chemists continue to alter the chemical makeup of synthetic opioids, temporarily avoiding prosecution until regulators enact bans on those too.

Fentanyl and carfentanil, synthetic opioids that can be significantly stronger than heroin, represent some of the most pressing challenges for drug enforcement agencies. The Centers for Disease Control and Prevention (CDC) reported this month that fatal overdoses from these substances shot up 72 percent from 2014 to 2015.

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But as drug policy officials continue to collaborate on enforcement strategy, there's hope that what happened to synthetics like K2 and Flakka will happen to fentanyl and carfentanil after China imposes new bans. It's a temporary solution, but at least in the short-term, it could mean less deadly drugs on the streets.