Students tortured teachers and beat them to death. Workers attacked one another with screwdrivers devised into spears. Temples and libraries were razed. Millions of people were banished to the countryside for "re-education."

The turbulence and viciousness of China's Cultural Revolution began 50 years ago, on May 16, 1966, with a Politburo decision to create a Cultural Revolution Group that would oppose "counter-revolutionary revisionists" and create a final rupture with the old ways of the capitalist past.

Two weeks later, a People's Daily editorial called for an attack on the bourgeoisie. "Sweep away all monsters and demons!" the state newspaper urged.

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The directive launched years of social disorder, factional warfare and even cannibalism. In total, between 1.5 million and two million people died, historians estimate.

But unlike the calamities perpetrated by Stalin, Pol Pot, Franco and Hitler, the regime responsible for the Cultural Revolution remains in power. The 50th anniversary will barely be discussed in public. The Communist Party has largely maintained a historical blackout, in hopes of suppressing the blemish on its legacy. "Researchers cannot accept any interviews related to the Cultural Revolution," one Chinese academic said this weekend.

That has presented a unique challenge to those documenting the Cultural Revolution – which is why the package sent to Song Yongyi 16 years ago, containing classified military plans in the city of Tianjin in the late 1960s, was so exceptional.

A Chinese-born librarian at Dickinson College in Pennsylvania, Prof. Song had only months before been released from his home country, where authorities had detained him on suspicion of smuggling secret documents. He had been on a quest to collect material on the Cultural Revolution, scouring archives and flea markets for copies of Red Guard newspapers. He wanted to compile a full record of the propaganda used to stoke tumult that, in some places, rendered indistinct the line between human and animal.

His time in detention looked like it would permanently end that quest. Instead, it gave him a global profile as someone determined to reveal the secrets of one of modern China's darkest hours.

The package that he received from a Chinese high school teacher became the first of many he would acquire in the years that followed, placing him at the forefront of a small band of researchers who have dug for documents in garbage dumps, archives, private collections and blogs. They have accepted information from unannounced visitors, spent long hours digging through archival documents and sought out personal accounts of hardship.

In the process, they have brought to light a stunningly detailed account of what happened during the Cultural Revolution, when Mao Zedong subverted normal societal order, orchestrating a mass lawlessness that saw factory workers imprison bosses and students torture teachers with boiling water and nail-spiked clubs.

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One of Prof. Song's signal achievements will emerge next month, when he begins the republication, in e-book form, of a 36-volume secret report on the Cultural Revolution in the country's southern Guangxi province.

Among the most startling revelations in its 13,000 pages is the cannibalism it documents. Whipped into a fury by the chaos of the times, Red Guards – groups of youth dedicated to removing enemies through violent class struggle – and others feasted on the hearts, livers, penises and breasts of people deemed "class enemies." In total, 421 were eaten in at least 31 provincial counties, Prof. Song said.

"So there was a cannibalism mass movement in the remote countryside."

Only now, 50 years after the beginning of the Cultural Revolution, has the account of that period been assembled in a form that anyone can read. It took six years of effort by Prof. Song to secure each volume from a network of archivists, collectors and high-ranking Chinese officials who smuggled out documents in hopes of exposing the truth.

The report forms "the most important information on the Cultural Revolution that exists today," said Jean Hung, who from 1988 to 2007 oversaw collection development at the Universities Service Centre of the Chinese University of Hong Kong.

If modern China has sought to suppress history, then, it has failed. Academics who study Soviet times, as one example, "tell me that the USSR was never so open about their dark historical past as China has been," said Andrew Walder, a Stanford political sociologist who has written extensively on the Mao era. That is, in part, due to efforts by elements inside China's Communist Party itself to internally document the ugliness Mao perpetrated.

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Few documents exemplify that like the Guangxi report, which was compiled in the 1980s at the direction of reformist elements inside Beijing who "wanted to see matters like the Cultural Revolution, these horrible things, get investigated," said Prof. Song, who is now a librarian at UCLA.

The documents list "who was eaten. Who was eaten by who. The reason," Prof. Song said. "My view is they expose the truth of Guangxi during the Cultural Revolution."

Far more information, however, remains locked away in Chinese archives, compiled by a one-party state with a "desire to tease out information about everything and everybody – and an attempt also, clearly, to keep all of this secret," said Frank Dikotter, a historian at the University of Hong Kong who wrote The Cultural Revolution: A People's History.

To research his book, Prof. Dikotter spent six months in Chinese archives. Some wouldn't let him in, or served tea before sending him on his way. Others, however, provided him documents that helped to peel back time.

Even mundane economic reports revealed the scale of dislocation beginning in 1966, as reverence for Mao grew feverish while people attacked and killed each other in efforts to wipe out "capitalist roaders." Entire categories of people lost work overnight, including florists, fruit sellers, undertakers and dressmakers.

Toymakers curbed output as plastic became needed, instead, to make glossy covers for the Little Red Book. Factories churned out 50 million Mao badges a month, but still struggled to keep up with extraordinary demand for wearable images of the leader. So much aluminum was diverted to those badges that by 1969 Mao himself had to step in, demanding "give me back my airplanes."

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Other documents detailed the chilling treatment of ethnic Mongolians in Inner Mongolia. "Tongues were ripped out, teeth extracted with pliers, eyes gouged from their sockets, flesh branded with hot irons," Prof. Dikotter wrote.

In another province, one in 50 people was accused of engaging in crime. "These crimes range from poking a hole inadvertently in a Mao poster to buying an egg on the black market," he said. With money diverted to Cultural Revolution efforts, and the country's health ministry attacked for serving leaders rather than the people, a meningitis outbreak spiralled out of control. Some 160,000 people died.

In Beijing, meanwhile, students killed dozens of teachers in a battle against "capitalist intellectuals." Roughly one in 100 teachers died by suicide. One used scissors to cut his throat; another used his glasses. One leapt into a vat of glue used to post "big character posters" that denounced adversaries. Another hit his head with a hammer. Details of their deaths emerged from extensive research by Youqin Wang, the Chinese language program director at the University of Chicago, who has compiled them on a website called the Chinese Holocaust Memorial.

Two other massive online Cultural Revolution repositories have also been built: an Internet-connected database of roughly 40,000 documents that Prof. Song helped to create, and another website run by University of Toronto researcher Yiching Wu that contains more than 10,000 texts and articles. Many of them are personal stories collected from online posts by Prof. Wu who preserves them from censorship in China.

The sheer volume of available documents stands as a testament to the ingenuity of those dedicated to revealing what happened.

Still, where the Chinese Communist Party has been unable to block preservation efforts it has succeeded in halting discussion. Inside China, few are willing to write on the Cultural Revolution, knowing their work won't be published. Outside China, the number of researchers working on the period can be mostly counted on two hands. Few people are actually reading the documents that have been compiled.

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"We do have lots of information. But that doesn't mean we know or understand," said Prof. Wu, who wrote The Cultural Revolution at the Margins: Chinese Socialism in Crisis.

Ms. Hung compares that to the Holocaust, which is the subject of entire libraries. The Cultural Revolution may have killed fewer people than Nazi Germany. But it scarred many more.

"This is really a pathetic situation," she said. "The Cultural Revolution, my goodness, lasted for 10 years. …The impact is immense, it's incomparable. Almost every single family was impacted."