Federal agents have raided warehouses in Mundelein and Arlington Heights and seized enough fentanyl to “supply a major city,” officials announced.

The Drug Enforcement Administration (DEA) said that Liangfu Huang and Wei Xu are facing federal drugs charges for illegally selling fentanyl and fentanyl precursors over the internet.

Authorities said that Huang used his company, Ark Pharm, to sell controlled substances, including a fentanyl precursor, over the internet without registering with federal or state authorities.

Ark Pharm, which is based in Arlington Heights, offered various drugs for sale on its website. Huang, 53, of Northbrook, was arrested Wednesday night after arriving at O’Hare Airport on a flight from China.

In a related case, Xu, 52, of Vernon Hills, used her company, 1717 CheMall, to illegally sell fentanyl and other controlled substances without registering with federal or state authorities. 1717 CheMall, which is based in Mundelein, offered various drugs for sale on its website including the opioids ocfentanil, oxycodone and hydrocodone. She was arrested on Wednesday.

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The DEA said that agents searched Ark Pharm and 1717 CheMall warehouses and seized their websites following a lengthy undercover investigation.

“Synthetic opioids killed 20,000 Americans in 2016–more than any other kind of drug,” said Attorney General Sessions. “The vast majority of these drugs originated in China and then either shipped through the mail or smuggled across our porous Southwest Border,” Sessions added.

Huang and Xu, who are both facing up to 20 years in prison, appeared before U.S. Magistrate Judge Mary M. Rowland, who ordered them to remain in federal custody. A detention hearing is scheduled for June 5.