WASHINGTON — Nearly $800 million worth of fentanyl pills were illegally sold to online customers in the United States over two years by Chinese distributors who took advantage of internet anonymity and an explosive growth in e-commerce, according to a Senate report released on Wednesday.

A yearlong Senate investigation found that American buyers of the illegal drugs lived mostly in Ohio, Pennsylvania and Florida. The Chinese sellers primarily used Bitcoin, the digital currency, as their preferred method of payment and shipped the drugs through other countries to reduce the risk of the opioids being seized by customs officials, Senate investigators said.

The 104-page bipartisan report was produced by the Senate Homeland Security Committee’s investigations arm. It was requested by Senators Rob Portman, Republican of Ohio and the panel’s chairman, and Tom Carper of Delaware, the committee’s top Democrat. They will discuss the report’s findings in a hearing set for Thursday and are expected to press law enforcement and Postal Service officials for details on plans to combat the growing opioid shipments.

China has a large pharmaceutical industry and hosts thousands of illegal labs that manufacture counterfeit and illicit drugs, Senate investigators said. The country has long been identified by American officials as the source of much of the illegal opioids flowing into the United States.

The Senate investigators said drugs traffickers are exploiting a loophole in the international mail system to send hundreds of pounds of deadly opioids into the United States each year.

But customs officials say they currently do not receive advanced shipping data from all packages shipped through Postal Service that can help the agency spot opioids hidden in international packages. The service is not required to obtain advanced data from foreign postal organizations; mailed packages are inspected manually by customs officers at nine mail facilities in the United States that receive international mail.

Congress is considering legislation that would require foreign post offices to provide that electronic information, but the Postal Service said many poor countries are unable to produce the data.

But even with the advance data, some drugs get through, federal law enforcement officials said. Drug trafficking organizations continue to use FedEx and UPS to send heroin and opioids into the country, according to investigators with Homeland Security Investigations, a division of Immigration and Customs Enforcement.

Investigators said synthetic drug traffickers are getting more creative in how they ship drugs through the postal system, exploiting the sheer volume of international mail and sending smaller packages.

According to federal investigators, the use of illegal fentanyl and other opioids in the United States has exploded since 2014. Nearly 43,000 Americans died from opioid overdoses in 2016, the most recent data available, according to the Centers for Disease Control.

Last October, President Trump declared the opioid crisis a public health emergency but did not make any new funding available to combat it.

On Monday, Patrick Kennedy, a former Democratic representative from Rhode Island and a member of Mr. Trump’s commission to address the opioid crisis, said the administration’s efforts were “essentially a sham.”

Nearly two weeks ago, Mr. Trump signed legislation to provide Customs and Border Protection officers with additional screening devices and other tools to detect illicit drugs.

The legislation provides $9 million to the agency to buy portable chemical screening devices and new laboratory equipment at land-based ports of entry, airports and international mail facilities.