But something the author found most disconcerting was the ambivalence Karen encountered while interviewing a number of Germans.

Architects of Death: The Family Who Engineered The Holocaust (Biteback, £20) — from which we published an extract last week — sees her delve into the history of J.A. Topf and Sons.

The company’s owners and senior engineers competed with each other to develop the technology for Nazis to carry out the Holocaust.

Journalist Karen told me: “The ambivalence I discovered was quite shocking.

“Many of the Germans I interviewed agreed, superficially, that what Topf and Sons did was wrong, but then they offered a qualification for it.

“They see them as only partly guilty because the company didn’t really make any money out of it or because they were ‘just doing their jobs’.

“They are looking to make an excuse for what happened.”

Topf and Sons, which was based in the city of Erfurt, started working with the Nazi regime on May 17, 1939.

Engineer Kurt Prüfer produced a drawing for a mobile, oil-heated Topf cremation oven, which secured the company’s first commission with the SS.

The mobile ovens were used to incinerate the growing number of bodies at concentration camps, including the nearby camp at Buchenwald.

And, by late 1941, Topf and Sons produced mobile and static single and double-muffle ovens for four Nazi concentration camps and designed a series of triple-muffle ovens to meet the demands of the SS at Auschwitz.

“The Topfs were not antisemites, but they were swept along with the Nazi machine,” Northumberland-raised Karen said.

“They were not Nazi ideologists at all, but they joined the Nazi party because they needed to gain control of the company from the other directors.

“When there were outspoken antisemitic comments at meetings, the Topf brothers said it did not want those sort of divisive comments.

“When you research the SS guards in concentration camps, for example, a lot of them were psychopaths and sadists, but still would have been had the Nazis not come to power.

“With Topf and Sons, they were office workers and engineers and they didn’t particularly believe in the Nazi cause, they just thought it was to their opportunity.”

The fact of the matter is, however, that, as well as the ovens, Topf and Sons’ fitter Heinrich Messing installed exhaust fans in the Auschwitz II crematoria and in the gas chamber, as well.

“By the end of war, Topf and Sons were suggesting designs so outrageous that even the Nazis couldn’t put them into practice,” explained London-based Karen.

“They came up with an idea for a circular oven where no external fuel would be used.

“Essentially, it would have meant bodies fuelling the burning of other bodies. They are nightmarish designs when you see them.”

Karen first came across the Topf story when she was writing After Auschwitz with Eva Schloss, Anne Frank’s stepsister.

“I was researching Auschwitz and came across a particular paragraph about Topf and Sons,” she recalled.

“I found it amazing how this company could have done such a terrible thing and I wanted to know more.”

She was put in contact with Erfurt University’s German history professor Rüdiger Bender, Hartmut Topf, a descendent of Topf and Sons’ owners, and Wolfgang Nossen, the leader of Thuringia’s Jewish community.

“Hartmut was instrumental in uncovering the truth about what happened,” Karen added.

“The family’s reputation has been horrendously tarnished, but Hartmut is the one person who has taken responsibility for what happened.

“He speaks the truth about it, something which nobody else in the family has done.”

On the Topf and Sons’ site today sits a place of remembrance.

It is the only memorial of its type relating to a civilian company’s collaboration in the Holocaust.

“I don’t think that exists anywhere else in Germany — there is no memorial at the place in Hamburg where Zyklon B was made, for example,” Karen said.

She is also working on a documentary with director Amna Khwaja about the life and work of Harmut.

And, at the moment, they are raising funds to complete the edit.

Karen, who is not Jewish, first learned about the Holocaust at a young age.

“My mother was always interested in it and when an Anne Frank exhibition came to Newcastle I went to see it, so that was the first time I became really aware,” she recalled.

“It is absolutely essential everyone remembers the Holocaust. Even to this day, there is a refusal to condemn antisemitism.

“I hope the book will continue to make people think about right-wing ideology.

“When it comes to antisemitism, I have been shocked and alarmed by the debate which has been going on in the Labour Party.

“It is always the right time to talk about antisemitism and remind people what happened.”

* tinyurl.com/TopfJT