How do you know the defendants engaged in those things? You’ve now remembered it 70 years later but were saying nothing 5, 10, 20 years afterwards?
 

For example, here is digitized version of a Guardian newspaper article dated October 2, 1945, reporting on the testimony of Dr. Charles Bendel:

https://www.theguardian.com/century/1940-1949/Story/0,,127738,00.html

From that article:

The prosecution put forward an unexpected witness in Charles Bendel, a Rumanian doctor resident in Paris for many years, who was arrested there in 1943 for not wearing the yellow star and was sent to Auschwitz. Here he worked at a crematorium as doctor to the special Kommando of prisoners required to dispose of bodies from the gas chamber and who were kept under lock and key lest details of what went on became known in the rest of the camp.
 

At eleven o'clock in the morning, witness went on, the chief of the political bureau would arrive on his motor-cycle to say that a new batch of prisoners had arrived. There would be between 800 and 1,000 of them, some on foot and others, usually too ill to walk, in tip-up trucks, from which, to the amusement of the drivers, they were spilled out without warning. They were taken into the courtyard or, in winter, to a large hall of the crematorium and required to undress under the pretext that they were to take a bath followed by hot coffee.
 

At Berkenau, a sector of the main camp, there were, stated witness, four crematoriums, each equipped with two gas chambers. But their capacity became insufficient and an additional system was devised by digging large trenches, some 12 yards by six, in which the bodies were burned, and to expedite matters channels were cut at the bottom of the trenches to drain away the human fat.

It was part of his Kommando's work to prepare these pits each day by laying huge wood fires saturated with petrol, and by this system it was possible to dispose of one thousand bodies an hour as against one thousand a day in a crematorium.

At eleven o'clock in the morning, witness went on, the chief of the political bureau would arrive on his motor-cycle to say that a new batch of prisoners had arrived. There would be between 800 and 1,000 of them, some on foot and others, usually too ill to walk, in tip-up trucks, from which, to the amusement of the drivers, they were spilled out without warning. They were taken into the courtyard or, in winter, to a large hall of the crematorium and required to undress under the pretext that they were to take a bath followed by hot coffee.

Then came a wait for the gas to arrive, said the witness. Raising his voice, he exclaimed that for a doctor the greatest insult was to have the cylinders delivered in a Red Cross ambulance in which, on occasion, he had seen the prisoner Klein.

Then the doors of the two-roofed gas chambers would be opened and the victims, who by this time realised that they were going to their death, herded in with blows from whips and sticks. Finally the guards would succeed in locking the doors. "You would hear cries and shouts as the people within began to fight against each other and to beat at the walls. This goes on for two minutes-then complete silence, nothing more."

About twenty minutes later, said Dr. Bendel, the main task of the special Kommando began. With the opening of the doors the bodies, tightly jammed inside, would fall out. Often it was almost impossible to separate one from another and one had the impression that they had fought terribly against death. Anyone who had seen a gas chamber filled to a height of four feet with corpses would never forget it. before they were dragged by their wrists to the ghats, barbers and dentists would shear off the hair and go over the teeth.
 

https://www.nationalarchives.gov.uk/education/resources/holocaust/gas-chambers-crematoria-birkenau/

From that affidavit:

There were two gas chambers, underground, roughly 10 meters long, 5 meters wide, and 1 1/2 meters high, each one. These two chambers supplied corpses to crematorium 1 and 2.

For crematoriums 3 and 4 there were two other gas chambers measuring each one 6 meters long and 3 meters wide and 1 1/2 meters high.

In the case of gas chambers for crematoriums 1 and 2 the gas was poured in through the roof. In the case of the gas for crematoriums 3 and 4 the gas was put in through small windows in the wall.

At BIRKENAW the gas used for killing people was prussic acid, also called ZYKLON B.
 

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