U.S. Congressional investigators said Wednesday that Chinese opioid manufacturers are exploiting weak screening at the U.S. Postal Service to ship large quantities of illegal drugs to American dealers.

In a yearlong probe, Senate investigators found that Chinese sellers, who openly market opioids such as fentanyl to U.S. buyers, are pushing delivery through the U.S. postal system. The sellers are taking advantage of a failure by the postal service to fully implement an electronic data system that would help authorities identify suspicious shipments.

At a time of massive growth in postal shipments from China due to e-commerce, the investigators found that the postal system received the electronic data on just over a third of all international packages, making more than 300 million packages in 2017 much harder to screen. Data in the Senate report shows no significant improvement during 2017 despite the urgency.

The U.S. Postal Service said it has made dramatic progress in the last year in total packages with opioids seized by U.S. Customs and Border Protection.

"The Postal Service will continue to work tirelessly to address this serious societal issue," spokesman David Partenheimer said in a statement.

He said implementing the use of electronic data is slowed by the need to negotiate with international partners, but the service is making progress.

Carfentanil, 100 times stronger than fentanyl, has been responsible for overdose deaths in the U.S. 2:25

The Senate probe matches many of the findings of a 2016 investigation by The Associated Press that detailed unchecked production in China of some of the world's most dangerous drugs.

AP reporters found multiple sellers willing to ship carfentanil — an opioid used as an elephant tranquilizer that is so potent it has been considered a chemical weapon. The sellers also offered advice on how to evade screening by U.S. authorities.

Researchers on the Permanent Subcommittee on Investigations also contacted Chinese sellers directly. The sellers preferred payment in bitcoin.

Investigators traced the online sellers to seven U.S. opioid deaths and 18 drug arrests. The Senate has cleared the report to be handed over to law enforcement.

In one case, the investigators traced orders from an online seller in China to a Michigan man who wired $200 in November 2016. The next month he received a package from someone identified by the investigators as a Pennsylvania-based distributor. A day later, the Michigan man died of an overdose from drugs, including a chemical similar to fentanyl.

The huge influx of opioids has led to a wave of overdose deaths across the U.S. in recent years. Republican Sen. Rob Portman, the subcommittee's chairman, noted that fentanyl now kills more people in his home state than heroin.

"The federal government can, and must, act to shore up our defences against this deadly drug and help save lives," he said.​