As the controversy grows over the authenticity of diaries allegedly written by Adolf Hitler, the frustrating quest for truth has focused on the shadowy source for more than 6,000 pages of black ink jottings and drawings.

The outside world seems excited by the historical mystery, but the West German public has reacted with a mixture of apathy and bemusement.

After months of reflection on the 50th anniversary of Hitler's rise to power, they seem inured to the prospect of an impassioned debate over the Fuhrer's penmanship.

The skeptics' view of the diaries' legitimacy was buttressed today when Joachim Fest, an esteemed biographer of the Nazi dictator, said he was offered the same material in the past by a collector of Hitler memorabilia identified only as an industrialist living in Stuttgart.

But the journalist who acquired the diaries, Gerd Heidemann, adamantly refuses to disclose how he got them and whom they came from because, he says, "people's lives are at stake."

For Peter Koch, editor-in-chief of the West German weekly Stern that jolted the world with what is either the hoax or the scoop of the decade, professional reputation is also in jeopardy. He admits that only Heidemann really knows who furnished the documents.

"I can't tell you where the diaries came from because I don't even know," Koch said in an interview. "We asked Heidemann many times but he refused to tell us. We respect his desire to keep it secret because we never had any reason to distrust him. He has worked here for 30 years."

Initially dubious about rumors of the diaries, Koch said he became convinced that they existed when Heidemann found the site of a crashed plane near Boehnersdorf, East Germany. Historians believe the plane was carrying Hitler's personal papers to a Bavarian retreat in April 1945. The journalist took photographs of the plane's debris and pilot Friedrich Gundlfinger's grave.

After Heidemann tracked down the diaries--Stern refuses to say how long the search lasted--Koch said he was persuaded they were authentic by the sheer bulk of the books.

"It's an enormous amount of work to forge so many volumes," he explained. "And why would Hitler's name appear at the bottom of almost every page when fake signatures can be detected so easily?"

The London Sunday Times reported that Heidemann paid an undisclosed sum of money for the diaries to a former Wehrmacht (Army) officer who hid them for many years in a hayloft. There is also speculation that the magazine paid more than $3 million to a collector who bought the papers from the officer, who in turn was never told that Stern would get them.

Koch will not comment on either account.

"All I can say is that we paid a lot of money for the diaries and we will be paid back with our good reputation," he said. "All our critics will have to eat their words."

As pressure builds on Stern to submit the diaries to an independent panel of experts, Koch says he wants to publish them in three sets of installments over the next 18 months before relinquishing the papers to federal archives.

But Interior Ministry spokesman Wighard Haerdtl said it is "incomprehensible" that Stern would not allow an impartial assessment of the entire text since such a judgment would be in the magazine's own interest.

Meanwhile, the head of West Germany's archives, Hans Booms said today that eight facsimiles provided by Stern to his experts appear to be authentic, although only one sample--a Nazi party statement drafted by Hitler concerning Rudolf Hess' secret mission to England--came from the diaries.

Koch contends that "the risk of losing our exclusive is too great to give up control of the original material."

He said the magazine was planning a lawsuit against Newsweek for breaking a confidentiality pact by publishing diary fragments in this week's issue that it promised not to reveal. An attorney for Newsweek had no immediate comment on the report.

The New York Post, whose owner Rupert Murdoch reportedly bought British Commonwealth rights to the diaries for $400,000, continued to cover the controversy over the documents, but did not publish excerpts. A source at the New York Post said Murdoch did not purchase U.S. rights to the diaries because the purchaser would have had to follow Stern's lengthy publishing schedule which would not be convenient for a daily newspaper.

Iain Calder, president and editor of the National Enquirer, said his publication had pulled out of the bidding for U.S. rights because of the doubts about the authenticity of the diaries and because "a lot" of the material "is coming out anyway."

Stern's bombshell already has been challenged by many Hitler-era specialists who doubt that the diaries are genuine. On Monday, British historian Hugh Trevor-Roper and American Prof. professor Gerhard Weinberg, who were flown in by Stern to reiterate their earlier positive judgments, expressed new reservations and requested that a body of eminent scholars examine the diaries.

Fest, the noted biographer of the Nazi dictator, said in a telephone interview from Frankfurt today that he had received "an offer of the same material three years ago from the same source."

Fest would only identify him as "a man living in Wuerttemberg," but other scholars familiar with the source described him as an industrialist living in Stuttgart, the capital of the state of Baden-Wuerttemberg.

Fest said he examined the diaries but concluded that while some segments might be genuine, the overall set looked fraudulent so he abstained from making an offer to buy the papers.

"I don't believe a word of Heidemann's story," Fest said, describing the reporter's tale of searching through Europe and South America in pursuit of the documents as "a lot of ballyhoo, nothing more."

Fest cited other factors that nurtured his doubts. He said that Hitler had a pathological aversion to revealing his feelings, motives and character.

Other scholars, including West German Prof. Werner Maser and Briton David Irving, said they also had perused drawings and writings allegedly done by Hitler that Maser characterized as "amazingly well-imitated."

Both men said they believe that the forgeries originate in East Germany and are sold in the West to earn hard currency or influence political views about the Nazi era.

The American professor, Weinberg, agreed that fake Hitler works come from East Germany but cautioned that "surely there are better and easier ways for the East Germans to earn hard cash."