Prominent U.S. Democratic senators on Tuesday called on the Biden administration to crack down on a popular duty-free customs program after Reuters reporting revealed how drug traffickers use the streamlined entry system to sneak Chinese-made fentanyl chemicals into the country virtually unchecked.

In a letter addressed to Treasury Secretary Janet Yellen and Homeland Security Secretary Alejandro Mayorkas, the lawmakers urged the agencies to use their statutory authority to prohibit e-commerce shipments from entering the U.S. under the so-called de minimis rule, which allows merchandise valued under $800 to come into the U.S. duty free, and with minimal paperwork and inspections. This customs channel is widely used by retailers and online shopping platforms to ship foreign-made goods directly to U.S. consumers.

The four lawmakers — Elizabeth Warren of Massachusetts, Sheldon Whitehouse of Rhode Island, Richard Blumenthal of Connecticut, and Sherrod Brown of Ohio — wrote that they were “particularly concerned by reports that drug traffickers are abusing the de minimis exemption to smuggle illicit fentanyl and its precursor chemicals into the United States.” The letter made multiple references to a Reuters investigation of the smuggling tactic published earlier this month.

“To combat the opioid epidemic and protect families…I’m asking the Treasury Department and the Department of Homeland Security to use their authority to close this dangerous loophole,” Warren said in a statement to Reuters.

Nearly four million de minimis packages arrive on U.S. shores daily. About 60% of those parcels come from China. The vast majority contain cheap clothing, electronics and mundane household products. Proponents of de minimis say scaling back the program dramatically would disrupt e-commerce, raise prices for shoppers, and overburden customs officials by forcing them to clear hundreds of millions more packages through traditional channels.

But this flood of low-value imports makes it difficult for law enforcement to intercept packages containing chemicals used to synthesize illicit fentanyl, the powerful synthetic opioid that is driving the worst drug crisis in U.S. history.

The lawmakers’ action comes amid growing pressure in Washington to overhaul the de minimis rule. In September, President Joe Biden moved to strip de minimis eligibility from a host of Chinese imports. The four senators in their letter called Biden's effort “a first step,” but they said “additional actions are necessary.”

Republican and Democratic lawmakers have also introduced a series of bills that aim to rein in de minimis, particularly for shipments from China. But bipartisan cooperation has been elusive in a tumultuous election year.