With tensions over trade with China escalating, President Donald Trump said on Friday that he is “ordering” the U.S. postal service and express shipping companies to “SEARCH FOR & REFUSE all deliveries of fentanyl,” a deadly synthetic opioid. American officials have long blamed China for the influx of fentanyl and related drugs reaching U.S. borders — sent directly and trafficked via other countries — while federal efforts to limit (if not stop outright) their import have long been underway.

Fentanyl is 50 times stronger than heroin, and proving increasingly popular with American drug users. While the rate of overdose deaths from heroin has plateaued in recent years, the rate of overdose deaths related to synthetic opioids, including fentanyl, has been increasing — by 45 percent between 2016 and 2017 (from 6.2 to 9 deaths per 100,000 people). About 28,400 people died from overdoses involving synthetic opioids in 2017, according to the Centers for Disease Control and Prevention.

As part of a wide-ranging deal between Trump and Chinese leader Xi Jinping last December, China had reportedly pledged to designate fentanyl and similar drugs as controlled substances, subjecting those who sell them to harsh punishments and potentially slowing their flow into the U.S. Trump said Jinping had told him that China did not have a drug problem, because it could (and does) use the death penalty to punish drug dealers.

Trump has since argued that China has not held up its end of the bargain. On Friday, China’s narcotics regulator said the U.S. is politicizing the issue of Chinese fentanyl exports and “up-ending the facts for their own political necessities,” Bloomberg reported.

What are authorities doing to deal with drugs shipped by mail?

Congress and U.S. Customs and Border Protection, the main federal agency dealing with preventing contraband from entering the country, have been working to crack down on drug trafficking through the mail for years. But it hasn’t been easy.

A nearly yearlong bipartisan Congressional investigation, as detailed in a January 2018 report from Sens. Tom Carper (D-DE) and Rob Portman (R-OH), found that Chinese fentanyl sellers “operate openly on the Internet.” These sellers’ “preferred method of shipping is the U.S. Postal Service because the risk of seizure by Customs & Border Protection (CPB) is small and delivery is basically guaranteed.” The investigation discovered hundreds of online drug transactions and linked online sellers in China to seven synthetic opioid-related deaths in the U.S.

Part of the STOP (Synthetics Trafficking and Overdose Prevention) Act, legislation passed in 2018 to help combat the opioid crisis in the United States, mandates that the USPS collects information on all mail sent from China, including details on the sender and the package’s contents.

A USPS spokesperson said in an emailed statement that the agency is “aggressively working” towards keeping dangerous drugs from entering the U.S. from China and other countries. Per the CBP’s website, international mail is screened thusly:

Mail entering the United States from abroad first arrives at a United States Postal Service (USPS) Sorting Facility. The Postal Service then sends packages to U.S. Customs and Border Protection (CBP) for examination and to assess duties and taxes, if any is owed. CBP processing is required for civilian parcels as well as those sent from overseas military postal facilities (APO/FPO). If CBP re-sealed a package due to examination, colored tape with the words “Examined by CBP” would be used.

Despite a recruiting crisis and agents being reassigned to the border — limiting resources needed to deal with identifying contraband at mail processing centers across the U.S. — CBP has said it is “committed to dedicating its resources to thwart illicit opioid supply chains and networks” and that “an effort is underway” to use advanced technologies such as pollen analysis to help detect fentanyl more efficiently.

How effective can mail inspections ever be?

The amount of fentanyl seized by CBP has been steadily increasing over the last few years: the agency seized almost 34 pounds from 50 incidents in the 2016 fiscal year, 96 pounds from 221 incidents in 2017 and more than 136 pounds from 455 incidents in 2018. But the agency appears to be straining under the sheer volume of mail the U.S. receives.

In 2017, Robert Perez, then an acting executive assistant commissioner at CBP, testified before Congress that, in the absence of advanced data to help target suspicious packages, officers were required to sort through large bags or bins of parcels manually. (Perez is now listed as deputy commissioner for the agency on their website.) “This manual process, again coupled with the tremendous volume of inbound mail to the United States, creates a daunting task for CBP,” Perez’s written testimony stated.

The 2018 Congressional investigation said that CPB’s manual efforts to inspect packages was “inefficient and the equivalent to finding a needle in a haystack.”

Lawmakers also noted in another 2018 congressional report about combatting the opioid epidemic that “few international mail packages are physically inspected by CBP” and on average, port officers employed by the CBP “only inspect 100 of the 1.3 million inbound international packages that USPS handles per day.”

In a statement to the Wall Street Journal following Trump’s tweets on Friday, a FedEx spokesperson said that “FedEx already has extensive security measures in place to prevent the use of our networks for illegal purposes… we follow the laws and regulations everywhere we do business and have a long history of close cooperation with authorities.”

Also via the WSJ, from a UPS spokesperson: “UPS takes a multilayered approach to security and compliance to identify and prevent delivery of illegal fentanyl and other illicit substances.”

Is it enough to go after the supply network?

“It’s critical that we work more closely with China, the main source for drugs like fentanyl that enter our country, to demand that they cut off the drug supply, while we work at home to stem demand,” Sen. Carper said during Congress’ investigation.

Leo Beletsky, a professor of law and health sciences at Northeastern University and an expert on opioids, says it’s the latter strategy that will make a difference. Trump’s remarks are a “knee-jerk statement,” Beletsky tells TIME, adding that it’s a “laughable proposition that we could stop the majority or even a major portion of contraband [even if] we triple the number of inspectors looking for the mail and delayed mail by multiple days.”

Trump’s statement is a reflection of U.S. drug policy more broadly, Beletsky notes — with a focus more on dismantling drug trafficking networks than helping vulnerable Americans struggling with substance abuse. Even if it were possible to do what Trump is proposing, it would not put a dent in fentanyl-related deaths, Beletsky says. He argues that the government should instead be directing money and resources into initiatives like expanding Medicaid, or prevention and rehabilitation programs which could work to make people less likely to become dependent on drugs in the first place.

“We’re much better served in trying to address demand for those substances and not turning to constantly play a cat-and-mouse game,” Beletsky says. “The evolution of fentanyl has been driven by efforts to crack down on the supply of heroin [and] encourages drug trafficking organizations to create ever more compact and less detectable drugs.”

The Trump Administration has said it has made it curtailing opioid abuse a priority, including by reducing the over-prescription of opioids and increasing funding for opioid and pain research from $600 million to $1.1 billion.

Write to Sanya Mansoor at sanya.mansoor@time.com.