THAT "crucial witness" on the John F. Kennedy assassination who killed himself last week just before he was to be questioned by a House assassinations committee investigator did have something to say.

But what George de Mohrenschildt could have testified to was not "new" evidence at all and it had nothing whatsoever to do with a conspiracy.

In fact, he had already said what he had to say, and when his memory was fresh, in 1964 before the Warren Commission. He was one of commission's key witnesses and its printed record includes 118 pages of testimony from de Mohrenschildt and almost an equal amount from his wife, in all nearly 300 pages.

But the truth about de Mohrenschildt is more fantastic than the wildest assassination conspiracy theory. It is one of the incredible ironies of history that the paths of de Mohrenschildt and Lee Harvey Oswald should have crossed, as they did, a few months before Oswald killed Kennedy on Nov. 22, 1963.

In fact, de Mohrenschildt knew Oswald better than anyone else alive, except perhaps for Oswald's wife, Marina, from September, 1962, until April, 1963 - when de Mohrenschildt moved from Dallas.

It isn't at all difficult to believe that de Mohrenschildt had at one time or another worked for the CIA, and it is a matter of record that he was an agent for the French government in the United States during World War II when he was posing as a perfume salesman.

I can speak with some authority about de Mohrenschildt's life because he once asked me to write his life story and I spent many hours listening to him recount some of his bizarre, almost unbelievable episodes. I came across him when in 1964 I was working for a television network as a special investigator on what was to become an hour-long documentary on the John F. Kennedy assassination.

I was probably the first of many reporters who had "exclusive" interviews with de Mohrenschildt, and mine has a special flavor because I had to go to Haiti to find him.

I stayed with de Mohrenschildt and his wife in their lovely house which clutched the side of a steep hill overlooking Port-Au-Prince - and which was, not insignificantly, I suppose, within the compound where Papa Doc Duvalier then lived.We had to pass through heavily-guarded gates as we came and went.

I learned from him how he had eventually married three socially-prominent women, one of whom was a Philadelphia heiress. He told me how he had worked in U.S. missions in Yugoslavia, in Ghana, and how he has walked the length of Mexico with his fourth wife and two small dogs.

This woman, de Mohrenschildt's widow, was a White Russian who had been born the daughter of the director of the Chinese Far Eastern Railway, in Harbin, and who had danced in nightclubs in Shanghai and Tientsin under the name "Fomenko." Her career brought her eventually to New York's Rainbow Room.

De Mohrenschildt himself was born in 1911 in Mozyr, Byelorussia, the son of a land-poor nobleman whose family migrated to Poland in 1922. De Mohrenschildt got part of his higher education - he held two Ph.D.s - in Belgium, and eventually he came to U.S. - in 1938.

The tragedy of his life was the death of his two children by the former Wynne Sharples, of Philadelphia. Both died of cystic fibrosis, and de Mohrenschildt was active in founding the National Cystic Fibrosis Foundation and succeeded in getting Jackie Kennedy to serve as honorary chairman when she was First Lady.

HIS PROFESSIONAL training was as a petroleum engineer, and he went to Dallas in the early 1950s to search out and exploit oil leases.

He did not do well in Dallas, and, although he was a member of the influential Petroleum Club, he made his social life in the city's Russian emigre colony, some of whose members tried to help Lee and Marina Oswald when they moved to Dallas in 1962. It was then and through this world that de Mohrenschildt and his wife met the Oswalds.

De Mohrenschildt talked many times with an Oswald who had not talked that much to anybody else. They talked mostly about politics. They agreed on most things - they both hated the John Birch Society which was active in Dallas at that time, they both hated the FBI and they both admired John F. Kennedy and Castro.

It was the accidental conjunction of their lives, and not the fact that George was an acutve observer, that made him a valuable witness for the Warren Commission. Under thorough questioning, George was able to recreate the life of the Oswalds at a critical juncture in their history and to reveal the political content of Lee Harvey Oswald's mind as it was a few months before Oswald killed Kennedy.

At the commission's request, he reconstructed each of his meetings with Oswald. At the end of his lengthy interrogation, de Mohrenschildt was asked to make an appraisal of Oswald. He was condescending and said he didn't think that Oswald was very intelligent.

De Mohrenschildt was too self-centered, too arrogant, to be sensitive to the feelings of others, and I did not find him to be the kind of person who could give any insight into why Oswald killed Kennedy. It seems apparent that Oswald had not decided to do so at the time de Mohrenschildt moved to Haiti.

That is why it seems unlikely that de Mohrenschildt had anything more to tell anybody - especially in March, 1977, 14 years after the event, 14 disastrous years in de Mohrenschildt's personal life. His last job was as a teacher in a small college and that ended when he simply stopped turning up for work. For months before he took his life, he had talked about committing suicide to his friends who were still members of that Russian emigre group in Dallas. They took him to dinner, tried to stimulate his interest in life again. They failed and he was admitted for three months to the Parkland Hospital psychiatric unit in 1976.

Some of the reports on de Mohrenschildt's death said he "felt guilty." He may have done so in the last stages of his life, but for years after the Kennedy assassination he did not show any signs of remorse. It was his wife who had him committed to Parkland Hospital and she did so because he had begun to think he was being followed.The question is whether de Mohrenschildt was responsible in February and March of this year when he gave his last interviews to newsmen.

He most certainly did not end his life because he was about to be questioned by a congressional committee. He most certainly did not know that Lee Harvey Oswald was going to kill Kennedy. What he failed to realize was that the difference between his rambling fantasies and those of Oswald's was the core of murderous anger that lay at the center of Oswald.

De Mohrenschildt's diagnosis of Oswald's situation was that Oswald seemed "too tense." He told Oswald, "You should take a drink once in a while and go out to a party."

But Oswald and history had it otherwise.