Carfentanil has experienced a drastic resurgence across the U.S., causing hundreds of unsuspecting drug users to overdose. It is a weapons-grade chemical that the Drug Enforcement Administration says is 10,000 times more potent than morphine and 100 times stronger than fentanyl. DEA intelligence bulletins show the rise coincides with a recent crackdown by the Chinese government on the sale of precursors used to make fentanyl. The bulletins say that is likely leading traffickers in Mexico to use carfentanil to boost the potency of a weakened version of fentanyl. The surge comes even as fentanyl seizures and overall drug overdose deaths continue a multiyear decline.