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.

“Fentanyl is now killing more Americans than heroin or any other opioid, and we should use every tool at our disposal to keep it from entering the United States,” McCaskill, who is the top-ranking Democrat on the Homeland Security and Governmental Affairs Committee, said in a statement.

Other than the Postal Service, McCaskill and Johnson requested answers from the State Department and the Department of Homeland Security. They are asking how the different agencies share information with one another and asking the Postal Service how it can work to better identify packages that might be suspicious.

“The opioid crisis in this country has reached epidemic proportions, and it is a multifaceted problem that demands a whole-of-government approach to resolve,” the senators wrote in their letters.

Deaths from drug overdoses have been increasing during the last decade, reaching 30,000 people in 2015, according to latest-available data from the Centers for Disease Control and Prevention. The Trump administration recently declared the opioid epidemic a national emergency, and has asked Congress to appropriate funding for prevention and treatment.

President Trump told reporters during his recent trip to Asia that he would be discussing the opioid epidemic and the shipments with Chinese President Xi.

“A special emphasis will be placed on the new phenomenon, fentanyl ,destroying lives by the millions. We’re going to be focusing on it very strongly, the president and myself,” he said.