The key to the enigma of the Hitler diary hoax seems to lie with a balding, mustachioed merchant who, until a couple of weeks ago, operated a small basement storehouse here selling Nazi memorabilia before dropping out of sight with more than $3 million.

To most neighbors and cronies at the local See Tavern, he was known as Konrad Kujau, a taciturn businessman rarely seen more than once a week, usually in the company of a client eager to inspect his goods.

Yet to business associates, the same man was Konrad Fischer, reputedly a refugee who escaped from East Germany in the late 1950s but who still retained ties with powerful relatives who furnished him with precious artifacts and the lost diaries of Adolf Hitler.

During an interrogation that lasted until dawn Saturday, Gerd Heidemann, the 51-year-old reporter sued for fraud this week by his former employers at Stern magazine, finally surrendered the name and address of the mysterious man to whom he paid 9 million marks or $3.75 million on behalf of Stern.

But when Stern investigators arrived at the wood and stucco corner building at 22 Schreiberstrasse here Saturday morning, heavy steel shutters were drawn over the windows of Fischer's shop and his storehouse annex in back. No name was posted on his mailbox, only the word "militaria" in black gothic letters.

The search for the vanished salesman intensified today as the Hamburg chief prosecutor, Guenter Wittke, opened a full investigation of Fischer's role in the case of the bogus diaries. Stern also announced that it plans to turn over custody of all 62 volumes to expedite police work.

After confirming today that he bought the diaries from Fischer, Heidemann said in Hamburg that he received a telephone call from his mystery source yesterday, originating, or so the caller claimed, from a public booth in Eastern Europe.

According to Heidemann, Fischer said he was seeking to trace the origins of the forgery. He insisted that he "had passed along the money" but was uncertain where it finally ended up. Heidemann said that when he asked Fischer whether he was the forger, he "assured me that he was not and said he believed all along that the diaries were real."

Heidemann's account generated fresh skepticism. Stern publisher Henri Nannen said the magazine had checked Heidemann's earlier tale that Fischer's collaborators were influential relatives in East Germany, purportedly an Army general and a museum director. In fact, scoffed Nannen, the two men turned out to be a railway station porter and a museum caretaker.

Reuter news service reported in London that Stern magazine has agreed to return the initial $200,000 payment made for the British and Commonwealth publication rights of the alleged diaries. The announcement was made by News International, the parent company of the Sunday Times, which had agreed to pay $400,000 for the rights on condition that the diaries were genuine.

In his dealings with potential buyers, Fischer, alias Kujau, often emphasized his supply channels to East Germany that were supposedly shielded by his high-ranking family relatives.

August Priesack, a Nazi archivist living in Munich, said that Fischer told him that he managed to smuggle the diaries across the border in a briefcase because his brother, the alleged Army general, accompanied him in full dress uniform to the Friedrichstrasse checkpoint in Berlin to ensure safe transit.

Priesack also said that Fischer told him his brother had served until 1972 as director of the military archives in Potsdam, a position that gave him access to important documents of the Nazi era.

An acquaintance of Priesack, Stuttgart University Professor Eberhard Jaeckel, said he had dealt with Fischer's wares before but became highly suspicious about historical accuracy. Jaeckel, who has edited a book on Hitler's early writings and was searching for more genuine papers, said he examined some documents for possible publication and later confronted Fischer with clear evidence that they were forgeries.

"I studied these papers and saw that there were gross historical mistakes," explained Jaeckel, puffing his pipe in high dudgeon during an interview in his office. "He was sitting right there, right on that couch in February 1981, and I told him flatly that these papers were fakes."

"That's when he cited the plane crash of April, 1945," said Jaeckel, referring to the demise of a cargo plane believed to be transporting Hitler's personal papers out of the Berlin bunker to a Bavarian retreat during the last days of the war. It is said to have crashed in flames near Boehnersdorf in what is now East Germany.

"After telling that story, Fischer claimed that he was able to get the documents through his special connections in East Germany, and that he was absolutely certain that the papers were authentic."

The explanation that Jaeckel heard was virtually identical to the one that Heidemann apparently believed. But the paunchy, owlish reporter proved more vulnerable because of his private passion for Nazi paraphernalia and his desire to score a reporting coup.

"The guy was really sick, always thinking about the diaries and working constantly to try and get them," recalled Stern reporter Wolfgang Schanke, who is now staking out Fischer's shop in Stuttgart to see if he ever returns. "It blinded him to the basic need to check out his source and find if he was legitimate."

While admitting he was "naive and gullible to turn over so much money" to Fischer, Heidemann insisted that he should not be blamed for Stern's failure to seek proper authentication during the two years that the magazine held the diaries before publishing them.

Stern's contemporary history section, where Heidemann worked, was concerned by the initials "F.H." on the diaries' covers but later assumed it stood for "Fuehrer Hauptquartier" (fuehrer's headquarters.)

As Stern and Heidemann continued to exchange bitter salvos over the costly and embarrassing scam, the fruitless hunt for the missing link, Fischer alias Kujau, has proceeded with a disappointing dearth of clues.

Members of Stern's investigative unit, dispatched to the Stuttgart area to track down Fischer, admit that for the moment they are completely stymied.

The local criminal bureau has discovered that he was accused in 1975, under the name of Kujau, of illegal possession of firearms and ammunition. But charges were later dropped when authorities accepted his claim that he only collected "militaria" for his shop but did not sell weapons for illicit profit.

The forensic tests on the diaries have not yielded any telltale signs about who concocted them or where.

Several historians have suggested that the benign depiction of Hitler in the fake diaries could point to a ring of old Nazis, in southern Germany or abroad, who are interested in rehabilitating the dictator's name. An even simpler theory suggests that Fischer wrote the diaries himself.