Sens. Claire McCaskill, D-Mo., and Ron Johnson, R-Wis., have asked several agencies including the Postal Service to detail how they work to stop illegal drugs like fentanyl from coming into the U.S. from other countries.

Various reports have show that overdose deaths from the opioid fentanyl, which is more potent than heroin, have nearly doubled from 2015, to 20,000 in 2016. Drug investigations have shown that fentanyl largely comes through the mail from China, and people who use or sell it often order it online.