Synthetic opioids such as Fentanyl are the top driver (20,145 fatalities) behind the unprecedented 64,070 drug overdose deaths past year, according to the U.S. Centers for Disease Control and Prevention (CDC).
The fentanyl comes from "labs" in China, which send the substance directly to those who consume it, or to middlemen that subsequently dilute the opioid for resale.
The investigation found that seven synthetic opioid-related deaths in the USA were linked to online sales specifically probed by the Senate subcommittee, The Guardian reported.
The probe did not mention the names of the suspected labs.
According to the report, the U.S. Postal Service has failed to widely deploy a system to capture advanced electronic data (AED) about packages destined for American ports, which would help identify suspicious mail to be turned over to U.S. Customs and Border Protection agents.
"The problem is the P.O. Box in America that gets a shipment through our postal service straight from China", the Ohio Republican told CBS News.
We are "prioritizing obtaining AED from the largest volume foreign posts, which collectively account for over 90 percent of inbound volumes", it added.
Investigators on the subcommittee followed up with six online retailers of fentanyl based in China after googling "fentanyl for sale".
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The investigation estimated that some 318 million parcels from overseas were not monitored as they were shipped via the US Postal Service past year.
Foreign ministry spokeswoman Hua Chunying reveals that "anti-drug coordination is one of the highlights of China-US law enforcement cooperation", as the Chinese government will "stand ready to work with the US to enhance our coordination in this field".
Nationally, drug overdoses are now the leading cause of accidental death for Americans under age 50, killing 63,600 people in 2016, according to data from the Centers for Disease Control and Prevention.
"We now know the depth to which drug traffickers exploit our mail system to ship fentanyl and other synthetic drugs into the United States", Portman said in a statement.
The investigation has also identified seven synthetic opioid-related deaths in the United States, and 18 arrests on drug-related charges, to Chinese sellers operating online.
The Senate Permanent Subcommittee on Investigations said that its investigators found through Google searches "hundreds of pages offering to sell" opioids, including highly-addictive fentanyl.
It is expected the report will now form the grounds for more stringent moves to combat the import of the drugs with more pressure on the postal service and border officials, the New York Times said.