It appeared to be a remarkable sting, finally bringing to justice one of the men who helped to kick-start the Holocaust.

Former SS Lieutenant Colonel Bernhard Frank, who had rubbed shoulders with Nazi leaders, was living out his old age in Frankfurt, never suspecting that the American neo-Nazi who had been interviewing him for years was really out to get him.

Last week, the 97-year-old found out. The American, Mark Gould, a self-proclaimed Nazi-hunter, had posed as a Nazi to get close to Frank and interview him.

Last week, he told the astonished 97-year-old man who he really was, and read out the civil suit he had lodged against Frank in a New York district court, for "compensatory, punitive and exemplary damages" for his alleged role in perpetrating the Holocaust.

But while some observers praise Mr Gould for his courage in pursuing Frank, the case has an experienced Nazi-hunter scratching his head, and some critics have questioned Mr Gould's motivations and historical accuracy.

Top Nazi-hunter and Israel director of the Simon Wiesenthal Centre Efraim Zuroff wrote in the Guardian that "Gould has clearly exaggerated Frank's role and importance in the context of Holocaust history."

According to Mr Gould's lawsuit, Frank, an aide to Heinrich Himmler, co-signed an order dated July 28 1941, with instructions to murder Jews in an area of the Ukraine where relatives of Mr Gould and his co-plaintiff, his cousin Burton Bernstein, were living. Twenty-four victims are listed, first names only.

The indictment also claims that Frank composed a directive, signed September 9 1941 by SS official Ernst Rode, to "exterminate ruthlessly" all Jews in areas where partisans were being sought.

Two historians agree that the orders of that period were pivotal in the course of the Holocaust. They "document the development of German genocidal policies in response to both real and imagined partisan threats," says Martin Dean, a research scholar at the US Holocaust Memorial Museum's Centre for Advanced Holocaust Studies.

"He did not create the order. He definitely signed it, 'Fur die Richtigkeit' - for correctness," says Stephen Smith, executive director of the USC Shoah Foundation Institute in Los Angeles.

"Gould has set a new precedent in legal history," Mr Smith said. "He has demonstrated that survivors and their families have a right to justice, with no time limit."

But critics say Frank's role was minor. They point out that he has never tried to hide his identity, even writing two autobiographical books, the most recent, As Hitler's Commandant - From the Wewelsburg to the Berghof, was completed four years ago.

The orders in question were "far from the first in which the murders of Jews were called for", writes British journalist Guy Walters, author of the 2009 book Hunting Evil: The Nazi war, about criminals who escaped and the dramatic hunt to bring them to justice.  

Mr Zuroff says that there is "no question that Bernhard Frank was an avid, zealous, passionate and committed Nazi.

"But it is one thing to expose his rabid antisemitism, and another thing to… put him at the top of the pyramid of criminality when in effect he was a very minor, marginal figure at best."

Besides, he said, one does not need to go to New York to prosecute an alleged German war criminal. "Gould could have gone to the German police. Germany is a country that puts Nazis on trial."