Deadly synthetic opioids are streaming into the United States amid a flood of mail that arrives unscreened from abroad every day, overwhelming the Postal Service and fueling the drug epidemic gripping much of the country, security experts and Massachusetts lawmakers say.

Nearly 1 million packages a day enter the country without any advance electronic information that might flag the presence of dangerous opioids such as fentanyl, much of which is manufactured in China, said Juliette Kayyem, a former assistant Homeland Security secretary.

An estimated 340 million pieces of mail each year pass unchecked through the Postal Service and US Customs without the prior electronic screening that was authorized by Congress 14 years ago but has yet to be fully implemented, Kayyem said.

The problem “is as disruptive to our country as any terrorist act has been’’ said Kayyem, who teaches at the John F. Kennedy School of Government at Harvard University.

In 2015, Massachusetts ranked second per capita in synthetic-opioid deaths, which include fentanyl, according to a new report from the Centers for Disease Control and Prevention. The number of such deaths in the state increased 109 percent from 2014 to 2015, the report said.

The scourge of fentanyl is still growing in Massachusetts. The drug has been found this year in three-quarters of overdose victims who had a toxicology test after their deaths, according to the state Department of Public Health.

Through September, as many as 1,475 opioid-related deaths had been confirmed or suspected in the state, a number on pace to break last year’s total of 1,759.

Much of the synthetic opioids that find their way into the United States — including fentanyl, which is 50 times more potent than heroin — originate in China but come through Mexico, where they are processed and packaged by drug cartels before being smuggled into the United States, federal officials say.

But a significant amount is bought by American users on the so-called dark web, a term for Internet sites that sell illegal products and then ship them directly from overseas and through the US mail, according to the experts.

For this, many buyers need no more than a computer, a click of a mouse, and delivery to their mailboxes.

The ease of using US mail to ship illicit drugs was tested in 2015 by the consulting firm LegitScript, which made 29 purchases from illegal online pharmacies, mostly in India. All 29 of the packages were delivered by the Postal Service after failing to be intercepted by customs.

Currently, much mail received from 191 foreign postal services, including China, Russia, and India, does not contain basic data such as the sender’s name, an address, and the package’s contents, according to Americans for Securing All Packages, a nonpartisan organization. (Kayyem serves as senior adviser for the group.)

Such data can be used to flag a package shipped from a suspicious or known illegal supplier. Once in the United States, officials said, the package stands a better chance to be plucked from the mail stream.

The national response to opioid-related deaths has been lagging, critics say. In 2002, Congress authorized the Customs Service to receive basic advance data on all incoming international mail. But since then, only private carriers such as Federal Express and United Parcel Service have conformed, Kayyem said.

“There is a lot of money to be made,’’ she said. In addition to opioids, security officials are concerned about hazardous materials and counterfeit goods that are slipping through.

David Parkenheim, a Postal Service spokesman, wrote in an e-mail to the Globe that the agency does receive advance electronic data on international shipments from “multiple countries, including China.’’

However, Parkenheim did not specify what details are received, how they are used, or what percentage of mail from large countries such as China, Russia, and India contain this information.

“The Postal Service shares the goal of those calling for expanding efforts to keep dangerous drugs out of the US mail system,’’ Parkenheim wrote.

To bolster screenings, Representative Richard Neal, a Democrat from Springfield, co-sponsored a bipartisan bill in September that would require advance electronic data — instead of merely authorizing the collection of such information — before packages enter the United States.

“This is one of those fascinating issues that unites Democrats and Republicans,’’ Neal said.

“It’s got the potential to equip the Postal Service and investigative agencies with the tools that they’re going to need.’’

Senator Edward Markey of Massachusetts, a Democrat who called fentanyl “the Godzilla of opioids’’ in an interview, also is working to curb the drug’s entry into the United States.

He and Senator Marco Rubio, a Florida Republican, secured approval of a resolution from the Foreign Relations Committee that calls on the US government to make the issue of illicit fentanyl “of the highest importance in our relations with China and Mexico,’’ Markey said.

“Donald Trump will need to understand that you cannot just build a wall to keep fentanyl out of America,’’ Markey said.

“We’re going to need a much closer working partnership with the Mexican and Chinese governments if we are going to be successful, and they need to understand that we mean business,’’ he said.

The senator said he also is crafting a bill to provide the Customs Service with high-tech equipment and more scientists to detect fentanyl on the spot.

Neal said the fentanyl trade probably is being promoted by gangs and criminal syndicates as part of China’s vast underground economy.

If his bill is approved, Neal said, he is hopeful that the Drug Enforcement Administration “and other investigative agencies here can work with their counterparts overseas to undertake this in more of a preventive manner.’’

In the meantime, the congressman said, “There is a long way to go.’’

Brian MacQuarrie can be reached at brian.macquarrie@globe.com.