China stands accused of a gruesome trade in human organs. It’s difficult to prove, because the victims’ bodies are disposed of and the only witnesses are the doctors, police and prison guards involved. Even so, the evidence supports a damning verdict.

The charge is that many prisoners of conscience—Falun Gong members, Uighur Muslims, Tibetan Buddhists and “underground” Christians—have been subjected to medical testing and had their organs forcibly removed. Those organs have fed an enormous trade in organ transplants.

Patients in China—including foreigners—are promised matching organs within days. Former Canadian politician and prosecutor David Kilgour, lawyer David Matas, American journalist Ethan Gutmann and a team of researchers have confirmed this by posing to Chinese hospitals as patients. Dr. Huang Jiefu, China’s former vice minister for health and chairman of its organ-transplant committee, ordered two spare livers as backups for a 2005 medical operation. They were delivered the next morning. In most advanced Western countries, patients wait months or even years for transplants.

In 2016 Messrs. Kilgour, Matas and Gutmann published a report, “Bloody Harvest/the Slaughter: An Update,” building on research that dates back to 2006. In this latest version, the authors estimated that between 60,000 and 100,000 organs are transplanted each year in Chinese hospitals.

Where are the organs coming from? China claims it has the “largest voluntary organ donation system in Asia” and stopped using prisoners in 2015. But the country has no tradition of voluntary organ donation.

In 2010 China’s official number of voluntary donors was 34. In 2018 China still had only about 6,000 official organ donors, who are said to donate more than 18,000 organs. Yet the “Bloody Harvest” researchers find that figure is “easily surpassed by just a few hospitals.” Tianjin First Center alone performs more than 6,000 transplants a year, and the report’s authors “verified and confirmed 712 hospitals which carry out liver and kidney transplants.” Dr. Huang claims China will perform the most transplants in the world by 2020—more than America’s 40,000 a year.

China’s figures don’t add up. To provide healthy, matching organs within days to patients at hundreds of hospitals, using only several thousand voluntary donors a year means there must be an additional, involuntary source of organs.

Death-row inmates cannot account for all of these. China executes more people than the rest of the world combined, but still only about a few thousand a year. Besides, Chinese law requires prisoners sentenced to death to be executed within seven days—not enough time to match their organs to patients and have them ready on demand, as is China’s practice.

That led the investigators to conclude that prisoners of conscience are the source of most of the mystery organs. The evidence is varied: Former prisoners of conscience have testified repeatedly that they were subjected to blood tests and unusual medical examinations in prison. The report claims the test results were then added to a database of living sources of organs, enabling transplants on demand—when a patient needs an organ, a prisoner of conscience from the list is harvested.

Falun Gong, a spiritual movement the Chinese government considers subversive, has been persecuted since a crackdown in 1999. In 2006 Chinese-speaking researchers posed as organ buyers and directly asked if organs from Falun Gong practitioners could be arranged for transplant. Hospitals throughout China confirmed they had such organs available, no problem.

The stories are brutal. Dr. Enver Tohti, a former surgeon from Xinjiang, testified in the British, Irish and European parliaments to removing organs from a prisoner forcibly in 1995. “We had been told to wait behind a hill, and come into the field as soon as we’d hear the gunshot,” he recalled. “A moment later there were gunshots. Not one, but many. We rushed into the field. An armed police officer approached us and told me where to go. He led us closer, then pointed to a body, saying, ‘This is the one.’ By then our chief surgeon appeared from nowhere and told me to remove the liver and two kidneys.” According to Dr. Tohti, the man’s wound was not necessarily fatal. But Dr. Tohti went ahead and removed the liver and kidneys while the man’s heart was still beating.

Experts around the world have testified to China’s crimes. Israel, Taiwan and Spain have banned “organ tourism” to China. United Nations rapporteurs have called China to account for the sources of their organs but received no response.

The Independent Tribunal Into Forced Organ Harvesting From Prisoners of Conscience in China is looking into the matter. Sir Geoffrey Nice, who prosecuted Slobodan Milosevic, chairs the panel of jurists and experts. On Dec. 10, they issued a rare interim judgment: The panel is “certain—unanimously, and sure beyond reasonable doubt—that in China, forced organ harvesting from prisoners of conscience has been practised for a substantial period of time, involving a very substantial number of victims” by the state.

The interim judgment was issued in the hope that it might “save innocents from harm.” If China has a response, I’d like to hear it.