Testifying before Congressman Christopher Smith and Dana Rohrabacher, investigators presented their findings.
According to the report, the tens of thousands of organ transplants not reported by the government, are sourced from executed prisoners of conscience who were locked up for their religious or political beliefs.
"That increased discrepancy leads us to conclude that there has been a far larger slaughter of practitioners of Falun Gong for their organs than we had originally estimated," the report read.
"The ultimate conclusion is that the Chinese Communist Party has engaged the state in the mass killings of innocents, primarily practitioners of the spiritually based set of exercises, Falun Gong, but also Uyghurs, Tibetans, and select House Christians, in order to obtain organs for transplants."
The authors claim that detained Falun Gong practitioners were forced to undergo medical tests before their results were put on a database of living organ sources so quick organ matches could be made.
Mr Gutmann said organ harvesting in China dated back almost 20 years when Falun Gong - a spiritual movement based on Chinese traditions - gained momentum.
"The Chinese Communist Party, alarmed at the growth of the (Falun Gong) movement and fearing for its own ideological supremacy banned the movement in 1999," Mr Gutmann said.
"Falun Gong practitioners were arrested in the hundreds of thousands and asked to recant. If they did not, they were tortured.
"If they still did not recant, they disappeared. Allegations surfaced in 2006 that the disappeared were being killed for their organs which were sold for large sums mostly to foreign transplant tourists. It is generally accepted that China kills prisoners for organs."
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 at a press conference on Thursday.
The National Health and Family Planning Commission, which oversees organ donations in China, could not be contacted for comment.
In 2005, Chinese officials admitted they harvested organs from prisoners and promised to reform the practice.
Five years later, director of the China Organ Donation Committee, Huang Jiefu, told medical journal The Lancet that more than 90 per cent of transplant organs were still sourced from executed prisoners.
In 2014, China announced that it would end the harvesting of organs from executed prisoners and move to a voluntary donation-based system.
But according to several reports, the controversial practice is far from abolished.
The Chinese Government has repeatedly refused to reveal how many people it executes each year.
China was named the world's biggest executioner in Amnesty International's Death Sentences and Executions 2015 report.
In releasing the annual report in April this year, the human rights group said it was impossible to obtain an exact figure on the number of people China has executed, but it is believed the figure is in the thousands, and is more than all the other countries in the world combined.
China was also named as the world's top executioner in 2014, with Amnesty estimating it was at least 1000 - a conservative figure, and one it believes is much higher.
However this year's report did note, there are indications that the number of executions has decreased since the Supreme People's Court began reviewing the implementation of the death penalty in 2007.
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.
