In early September 1939, Hitler’s legions swept into Poland and the Second World War began.

The JC, of course, reported on events closely, including the terrible treatment in those early weeks of Jews in the newly conquered Nazi territories. But perhaps the first indication of the extent of horrors to come came in a small item found in the October 20, 1939 edition of the paper. It showed a picture: a reproduction of a postcard written in Yiddish which had been sent to a neutral country from Berlin.

The paper provided a translation: “I appeal to you, help, help us. Men of Polish nationality who have been taken away return home as ashes. They are all slaughtered, young healthy people. Do all you can to make it known; write to all countries, to all newspapers; help us, save us — take us out.

“People have mercy. Shout it in the streets. Quickly, quickly.”

In the years to come there were a number of people who would do all they could to make the crimes of the Holocaust known. During the war itself, individuals and groups would repeatedly risk their lives to document what was happening, not knowing whether they themselves would survive to tell the tale. And then after the war, researchers worked tirelessly to uncover the terrible scope of the tragedy, for a variety of reasons — some attempting to trace the missing or dead, amass evidence in order to bring as many of the perpetrators as possible to justice, or even to develop a whole new branch of international law specifically concerned with human rights.

In its new exhibition, Crimes Uncovered: The First Generation of Holocaust Researchers, the Wiener Library looks in closer detail at some of these heroes. The exhibition has been produced in association with the House of the Wannsee Conference Holocaust Memorial, and funded by the German Foreign Ministry.

Barbara Warnock, the exhibition’s curator, said that among those featured are staff from the Wiener Library itself, including its founder, Alfred Wiener, who appears to have possessed an almost eerie degree of prescience. Among the items exhibited is a 1919 pamphlet he wrote, called Before Pogroms.

Ms Warnock said: “Wiener became concerned about levels of antisemitism in Germany. Before Pogroms warns that the antisemitism that existed then might lead to an orchestrated, violent attack on the Jews in Germany.

“He seems to have been quite foresighted. He was gathering information about antisemitism, the activities of extremist nationalist groups, but from the mid-1920s he became specifically very focused on the Nazis as a particular threat.” As war clouds gathered on the horizon in 1939, Wiener, worried that Holland would not escape the conflict, moved his organisation, which was dedicated to gathering evidence and information about Nazi activities, to London.

Yet despite warning for so long of the threat to come, Wiener did not escape the war unscathed.

“Wiener had a visa at that point but his family didn’t,” Ms Warnock said.

“They later got visas, but by the time that the Germans invaded Holland in May 1940 they hadn’t used them yet and they got stranded. His wife and three daughters ended up in Bergen Belsen. His daughters survived but his wife died on the day she was released.”

Emanuel Ringelblum and Rachel Auerbach were in a very different situation to Wiener. A historian by profession, Dr Ringelblum was among the hundreds of thousands of Jews trapped in the Warsaw Ghetto. He formed an organisation called Oyneg Shabes (‘The Joy of the Sabbath’).

As Ms Warnock said, the organisation “was about evidence gathering. It sought to collect information about what was happening in the ghetto, and also recorded testimonies and reports and accounts, with a view both to the historical record and to being a testament to Jewish life there.

“At the point that it was obvious that the ghetto was going to be liquidated, the organisation hid its collections in places, basements inside the ghetto, in metal boxes and tin cans.

“Ringelblum himself didn’t survive — he went into hiding, but he and his wife and his son were found and later murdered.”

Rachel Auerbach, however, did survive the Warsaw Ghetto, and was instrumental in the recovery of significant parts of the collection after the war.

The exhibition includes a copy of the means of her survival — “false papers, which gave her a non-Jewish name and identity”. It also contains a book she published in 1947, “recording early testimonies to do with Treblinka”.

If the work of Ringelblum and Auerbach was dangerous, the actions of Filip Müller were close to suicidal. Few people can be said to have done more, in the worst of circumstances, to ensure that the evidence of mass murder not be forgotten.

For more than two years, he worked at the gas chambers at Auschwitz as a sonderkommando — Jews who were forced, on pain of death, to aid with the disposal of victims.

Ms Warnock said: “He contributed to efforts that there were at times among the sonderkommando to gather evidence and pass it out, to try and tell the world about Auschwitz. He tried to keep records where he could of the names of SS involved at Auschwitz, details of transports.

"He also managed to get a label from a can of Zyklon B — and that information was all part of a cache of information that two sonderkommandos who successfully escaped managed to take out. They managed not just to successfully escape but then get right out of Occupied Europe, and they published a book called The Auschwitz Protocols.”

Also featured is a copy of a telegram sent in August 1942, by Gerhart Riegner, a German Jew who had moved to Switzerland prior to the war and became the Head of the World Jewish Congress office in Geneva. Sent via British and US diplomats to the Foreign Office and State Departments, the telegram was the first official warning of what would become known as the Holocaust:

“Received an alarming report stating that in the Fuehrer’s headquarters, a plan has been discussed, and is under consideration, according to which all Jews in countries occupied or controlled by Germany should, after deportation and concentration in the East, be at one blow exterminated, in order to resolve, once and for all, the Jewish question in Europe”, it read.

“Ways of execution are still being discussed including the use of prussic acid [Zyklon B].”

The telegram was all but ignored by the Allies. Featured with his picture is a haunting quote from Herr Riegner: “Never did I feel so strongly the sense of abandonment, powerlessness and loneliness as when I sent messages of disaster and horror to the free world and no-one believed me”.

Reports of horrors would become more public, however, later that year. The exhibition contains a pamphlet from December 1942, based on a statement from the Polish Government in Exile based in London.

It was called “The Mass Extermination of Jews in German-Occupied Poland”. As well as providing an in-depth description of actions the Nazis had taken against Jews since invading Poland, it also described how “the new methods of mass slaughter applied during the last few months confirm the fact that the German authorities aim with systematic deliberation at the total extermination of the Jewish population of Poland and of the many thousands of Jews whom the German authorities have deported to Poland from Western and Central European countries and from the German Reich itself.”

As Ms Warnock says, this declaration — and publication — “attracted press attention around the world. People often think that no-one knew until later on in the war. If you were paying attention to the press in late 1942, then you would have known that there weren’t just deaths occurring because of war, but that there was a systematic — the word ‘genocide’ wasn’t [yet] coined — campaign to murder Jews.”

The JC archive suggests as much. The paper’s front page on December 11, 1942 told readers: “Two Million Jews Slaughtered”.

Other items on display in the exhibition include the diaries of an inmate at the Theresienstadt ghetto, and a translation of the work of two Jewish Soviet journalists, Ilya Ehrenburg and Vassily Grossman, describing in detail the eyewitness accounts which had been given to them of Nazi atrocities against Jews.

There is also a 1954 Association of Jewish Refugees newsletter featuring an appeal for eye-witness testimony by Eva Reichmann, an eminent German historian who fled to London in 1939. Further individuals profiled include Hersch Lauterpacht, who would serve as a judge on the International Court of Justice; Louis de Jong, founder of the Dutch Institute for War Documentation; Rafael Lemkin, who was responsible for coining the word ‘genocide’ and initiating the UN Convention on the Prevention and Punishment of the Crime of Genocide in 1948; and Simon Wiesenthal, the well-known Nazi-hunter.

Few of the people who risked their lives to make sure the crimes of the Holocaust were made known could have imagined that, more than seven decades later, Holocaust denial and revisionism would be experiencing a resurgence.

Ms Warnock said: “It’s amazing that there could be denialism […] because of the weight of historical evidence, but it shows how it remains very important for people to understand what a well-documented historical event it is.”

The ‘Crimes Uncovered’ exhibition will run until Friday May 17