They blame the Chinese government, the Communist Party, the health system, doctors and hospitals for being complicit.
"The (Communist Party) says the total number of legal transplants is about 10,000 per year. But we can easily surpass the official Chinese figure just by looking at the two or three biggest hospitals," Matas said in a statement.
The report estimates that 60,000 to 100,000 organs are transplanted each year in Chinese hospitals.
According to the report, that gap is made up of executed prisoners, many of them prisoners of conscience locked up for their religious or political beliefs. China does not report its total number of executions, which it regards as a secret.
The report's findings stand in stark contrast to Beijing's claim that, since the beginning of 2015, China has moved
from almost completely relying on organs from prisoners to the "largest voluntary organ donation system in Asia."
At a regular press conference Thursday, Chinese Foreign Ministry spokeswoman Hua Chunying said China has "strict laws and regulations on this issue."
"As for the testimony and the published report, I want to say that such stories about forced organ harvesting in China are imaginary and baseless -- they don't have any factual foundation," she said.
The National Health and Family Planning Commission, which oversees organ donations in China, did not respond to a request for comment for this piece.
According to the report, thousands of people are being executed in China in secret and their organs harvested for use in transplant operations.
So who is being killed? The authors say mainly imprisoned religious and ethnic minorities, including Uyghurs, Tibetans, underground Christians, and practitioners of the banned Falun Gong spiritual movement.
While much of China's organ transplant system is kept secret, official figures
show that 2,766 volunteers donated organs in 2015, with 7,785 large organs acquired.
Official figures put the number of transplant operations at around 10,000 a year, which the report disputes.
The authors point to publicly available statements
released by hospitals across China claiming they carried out thousands of transplant annually, and interviews
with and official biographies
of individual doctors who claim to have carried out thousands of transplant operations during their careers.
"Simply by adding up a handful of the hospitals that have been profiled in this (report), it's easy to come up with higher annual transplant volume figures than 10,000," the authors write.
According to official statistics, there are more than 100 hospitals in China approved to carry out organ transplant operations. But the report states the authors have "verified and confirmed 712 hospitals which carry out liver and kidney transplants," and claims the number of actual transplants could be hundreds of thousands larger than China reports.
'Ghoulish and inhumane practice'
The apparent gap in official transplant figures, the report claims, is filled by prisoners of conscience.
According to Amnesty International, "tens of thousands of Falun Gong practitioners have been arbitrarily detained" since the government launched a crackdown on the practice in 1999.
China regards Falun Gong as a "cult" and claims followers engage in "anti-China political activities."
"The government considers Falun Gong a threat to its power, and has detained, imprisoned and tortured its followers," says Maya Wang, China researcher for Human Rights Watch.
The report says detained Falun Gong practitioners were forced to have blood tests and medical exams. Those test results were placed in a database of living organ sources so quick organ matches could be made, the authors claim.
This massive supply of organs served to benefit hospitals and doctors, making for an ever growing industry.
"The Chinese government has been trafficking in organs for profit for far too long and we have strong evidence that Falun Gong practitioners were singled out for organ harvesting," said Representative Chris Smith, who co-chairs the committee.
In a statement released online
, Representative Ileana Ros-Lehtinen, former chair of the U.S. House Foreign Affairs Committee, said the Chinese government's "ghoulish and inhumane practice of robbing individuals of their freedom, throwing them in labor camps or prisons, and then executing them and harvesting their organs for transplants is beyond the pale of comprehension and must be opposed universally and ended unconditionally."
For decades, Chinese officials strenuously denied that they harvested organs from prisoners, calling
claims to the contrary "vicious slander."
Finally in 2005, officials admitted that the practice took place and promised to reform it.
Five years later however, Huang Jiefu, director of the China Organ Donation Committee, told medical journal The Lancet that more than 90% of transplant organs still came from executed prisoners.
China carries out more executions annually than the rest of the world put together
, at least 2,400 in 2014, according to Death Penalty Worldwide. Official Chinese figures are not reported.
In late 2014, China announced
that it would switch to a completely voluntary donation-based system.
This pronouncement was greeted with great skepticism however, given that between 2012 and 2013, only around 1,400 people signed up to donate (compared to the more than 300,000 in need of organ transplants every year).
Since then, the government has seen limited success in getting people to sign up to the national register.
One 86-year-old woman, surnamed Zhou, told CNN she had wanted to donate her organs in 1996 but at the time her local Red Cross chapter had never heard of someone doing so.
"Since I wasn't able to have a medical career myself, I want to make a contribution after I die," she said.
Zhou said that while her family was mostly supportive of her decision, "in China, the conventional wisdom is that it's improper to mutilate a body when someone is dead."
While people like Zhou have stepped forward to fill the gap left by prisoners, experts warn that there is nothing to stop those condemned to be executed from also "volunteering," and regulations legalizing
the use of prisoners' organs remain in force.
The 2014 announcement "is only at best a statement of good intentions but has no force of law," the medical journal BMJ said
The phasing out of executed prisoners' organs is a "semantic trick," Professor Li Huige of Johannes Gutenberg University said in a recent report
commissioned by the European Parliament.
He pointed to statements by Huang to Chinese state media that "death row prisoners are also citizens."
"If (they) are willing to atone for their crime by donating organs, they should be encouraged," Huang told People's Daily
By redefining prisoners as regular citizens, Li says, "China's national organ donation system may be abused for the whitewashing of organs from both death row prisoners and prisoners of conscience."
In an open letter to the Lancet, five doctors wrote that "China is still using death row inmates' organs. The only difference is that these organs are now been classified as citizens' voluntarily donated organs."
Huang did not respond to a request for comment. Speaking to the New York Times
, he said his comments had been "distorted" and were not in keeping with government policy.
Testifying before the U.S. House Foreign Affairs Committee on Thursday
, Francis Delmonico, president of the Transplantation Society, praised Huang as a "principal ally to change the outrageous practice" of using prisoners' organs.