The file has little to say about Gerhardt. But its pages are crammed with details about a man Clea and I had never heard of: Paul Felix Nemenyi.

Born to a prominent Jewish family in Hungary in 1895, he was brilliant. At 17, he was co-winner of a national math and physics competition. He had a special gift for spatial relations -- a skill important to chess players. In the 1920s, he became a university lecturer in Berlin.

Nemenyi lost his position in 1933, when "non-Aryans" were purged from German universities. He settled in the U.S., and in 1942 he was teaching at what was then Colorado State College in Fort Collins. Regina was studying at the University of Denver.

They appear to have met and had an affair. Gerhardt, unable to obtain a visa to enter the U.S., was in Chile at the time, according to the FBI file.

In 1943, Regina moved to Chicago, alone. Bobby was born there in March of that year. His birth certificate lists Gerhardt as the father. FBI agents doubted that was possible, since their records indicated he had never set foot in the U.S.

Regina herself told conflicting stories about the father's identity. She was itinerant, shuttling between jobs and schools all over the country. She would often turn to Jewish social service agencies for advice and financial aid.

From employees at these agencies, the FBI learned that Regina once told a social worker that she hadn't seen Gerhardt since 1939, four years before Bobby was born. On another occasion, she said Bobby was conceived when she visited Gerhardt in Mexico.

Nemenyi visited some of these same agencies, according to the FBI file. He arranged to pay child support and told social workers he was unhappy with how Regina was raising Bobby. He once grew so upset describing his quarrels with Regina, according to one document, that he began "weeping."

On the Internet, Clea and I found an obituary of a civil rights activist from North Carolina named Peter Nemenyi -- Paul Nemenyi's son from an early marriage.

Peter, who died in 2002, was about 15 years older than Bobby and had told friends he was Fischer's half-brother. He had a doctorate in mathematics from Princeton University and supported himself teaching. Clea ran his name through a database of university archives and discovered that some of his personal papers were at the University of Wisconsin.

An archivist faxed us the file. It contained a letter that Peter wrote after his father died in 1952. Bobby was 9 at the time. Peter was writing to the boy's psychiatrist for advice. Regina had asked him to break the news of Paul's death to Bobby, and he wasn't sure how to do it. He had met Bobby only a couple of times.

"I take it you know that Paul was Bobby's father," Peter wrote.

The file also contained a letter to Peter from Regina. She said she was destitute because of Paul's death -- Paul had been paying for Bobby's schooling.

Clea and I wrote a story about these discoveries in 2002 for the Philadelphia Inquirer, where we both worked at the time. For a pair of Fischer obsessives, it was a heady feeling. We'd uncovered information that suggested the generally accepted outline of Fischer's life was wrong in one very important detail: His father was not Gerhardt Fischer, but rather Paul Nemenyi.

If our conclusion held up, it could change the popular understanding of Fischer and the formative influences on him.

Response to our disclosure was mixed. Friends of Bobby were skeptical. Fischer himself did not react publicly from his overseas exile: We sent him letters in Budapest and Tokyo, cities where he had been seen, but got no reply.

In subsequent articles, some writers referred to our findings about Nemenyi as a theory, nothing more.

So we kept at it. Clea went back online and discovered that letters Paul had written to the Hungarian-born engineer Theodore von Karman were stored at Caltech. She contacted the school in 2005 and it sent us copies.

Inside the package we were stunned to find a photograph of Paul Nemenyi -- the first we'd ever seen. He had enclosed it with one of his letters to Von Karman. The resemblance to Bobby was striking: the same deep-set, burning eyes, the same facial structure, even the same unruly shock of hair.

Next, we pulled Nemenyi's probate file from court archives in Washington, D.C., where he died. It contained a petition by the executor of Nemenyi's estate -- a lawyer hired by his son Peter -- stating that Bobby had been born to Regina and Nemenyi "out of wedlock."