WASHINGTON -- Synthetic opioids such as addictive and deadly fentanyl, fueling a drug epidemic, can be easily ordered online and shipped through the mail to United States addresses from China, a congressional investigation found.

The investigation under Sens. Rob Portman of Ohio and Tom Carper of Delaware pins blame on the U.S. Postal Service and State Department. The agencies have rebuffed requests to require electronic shipping information with names and addresses of senders and receivers of international parcels, the senators said. Portman, an Ohio Republican, chairs the Senate Permanent Subcommittee on Investigations, and Carper, of Delaware, is the panel's top Democrat.

The government agencies have cited logistical and foreign relations reasons for not policing international parcels the way Portman wants. Using the new investigation, Portman will demand answers again at a hearing Thursday morning.

Subcommittee investigators launched their investigation by using Google to look up websites for synthetic opioids. This led them to "hundreds of pages offering the stuff for sale," said one investigator, who spoke with reporters in advance of the hearing on the condition that he and others not be named.

Posing as first-time buyers, the investigators said they ultimately communicated with six "very responsive" sellers trying to market fentanyl -- the top source of accidental deaths in Ohio -- or other synthetic opioids.

The sellers offered "flash sales" and discounts when the investigators appeared to hesitate, and even tried to "upsell" by offering deals on carfentanil, which authorities say is 100 times more powerful than fentanyl.

The sellers offered several methods of payment, preferring Bitcoin because the cryptocurrency is hard to trace but also accepting payment through Western Union, MoneyGram, PayPal, credit cards and prepaid gift cards.

A message and answer between a Senate investigator asking about buying fentanyl and a willing online seller.

The investigators said they didn't buy the drugs. But using congressional subpoena power, they got payment and transaction records from Western Union and other money transfer agencies.

This helped them identify more than 500 financial transactions by more than 300 buyers in 43 states, totaling $230,000 to the six online sellers, according to a report of the investigation being released late Wednesday.

"Individuals in Ohio, Pennsylvania and Florida made the highest number of purchases," a subcommittee investigator said.

The shipments most often came from China, though some were sent first to an intermediary in Europe before being re-shipped to the United States.

Using information from the payment services, investigators were able to identify 18 buyers who were arrested and had dealt with the online sellers. They also found obituaries and death records showing that seven buyers died of fentanyl-related overdoses.

This included a 49-year-old Ohioan who sent about $2,500 to an online seller over the course of 10 months, receiving 15 packages from the Postal Service, the Portman-Carper report says.

The investigative subcommittee will turn its information over to law enforcement agency officials Thursday, using a process that requires Senate approval first. The Senate unanimously consented Tuesday.

Members of Portman's subcommittee say they decry the entire drug trade's existence. But the panel's jurisdiction is over United States government operations. That's why it has the Postal Service in its sights.

Private parcel services such as Federal Express, United Parcel Service and DHL were ordered by Congress after 9/11 to collect names and addresses of anyone shipping or receiving a package. That data, contained in electronic records, can be used by Customs and Border Patrol agents in investigations, including when it suspects drug crimes. Law enforcement agencies can combine information they receive from various sources, including detection of drugs in a package, to find other parcels -- if they have advanced electronic data to easily spot shippers and recipients.

But the Postal Service, with a much higher volume of shipments, got a pass in that legislation, which waived them from requiring advanced electronic data. Meantime the volume of potentially illegal shipments has grown. The volume of inbound international mail grew by 232 percent between fiscal 2013 and calendar 2017 alone, according to the subcommittee, potentially enabling an increasingly higher volume of drugs to go undetected.

Despite Portman's attempts to close the gap with a bill requiring electronic data collection for all parcels, postal officials have balked, saying the scope of requiring this would be huge. And postal and State Department officials told Portman at a hearing last May that they worried some foreign governments weren't technologically capable of complying -- and that a requirement could spark a backlash from some countries, slowing or halting U.S. mail sent overseas.

The Postal Service has worked with Customs and Border Patrol to hand over for inspection packages sent from countries of interest, identified as such by the customs agency. This is done at five international mail service centers including major airports in New York, Los Angeles and Miami.

But the investigative subcommittee report says this is "needle in a haystack" work and inefficient. And although the Postal Service and customs agency in 2015 began a pilot program at the JFK International Airport mail service center to use advanced data screening from certain countries that use such data with their international mail, it has had holes and the two agencies are on different missions, according to Portman's subcommittee report.

Besides, subcommittee investigators said, only 36 percent of packages sent to the United States in 2017 had advanced electronic data tracking information.

Drug dealers know and like those holes, the investigators said.

"Thanks to our bipartisan investigation, 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 federal government can, and must, act to shore up our defenses against this deadly drug and help save lives."

The investigation, Carper said, "has uncovered how incredibly easy it is to buy these deadly drugs online and have them shipped here through the mail. We have also learned how ill-equipped federal agencies were to prevent drug smugglers from taking advantage of a massive surge in recent years of e-commerce and international mail to ship synthetic opioids, like fentanyl, into our communities."

Answering Portman's and Carper's questions at the hearing Thursday will be officials from the Postal Service, State Department, Department of Homeland Security and Justice Department.

Investigators for the subcommittee said it is clear that opioid dealers appreciate the U.S. government's reluctance to step up screening. This was apparent in the dealers' desire to use the mail rather than a private service.

If the investigators wanted to use private parcel services rather than the mail, they said, they were told the dealers could not guarantee their fentanyl would arrive.