A Watergate-era mystery that remains unsolved to this day has convinced some of those involved that the cover-ups of the Nixon administration continue.

The mystery surrounds Elias Demetracopoulos, then a Greek exile leader in the United States. Were U.S. government officials involved in two plots to return him forcibly to Athens, where he faced possible execution? The notion of a cover-up arises from the fact that Demetracopoulos has obtained tantalizing evidence that numerous government files about his case exist. But everyone involved pleads ignorance.

One person who ought to be able to help is Henry Kissinger, national security adviser and secretary of state under Richard Nixon and Gerald Ford. But Kissinger has sent Demetracopoulos trudging through the voluminous archives of numerous federal agencies in search of scattered pieces of the puzzle.

Demetracopoulos, a journalist, escaped from Greece in September 1967 and set up shop in Washington five months after the military junta seized power. He lobbied furiously and ably against U.S. cooperation with the colonels and became a thorn in the side of Nixon and his pro-junta supporters.

In February 1975, we disclosed that Attorney General John Mitchell had discussed a proposal to deport Demetracopoulos to the colonels, who wanted him silenced. In the early '70s, Mitchell was a member of the National Security Council's ''40 Committee,'' a secret group chaired by Kissinger that reviewed covert operations. Two months later, we quoted Greek junta documents that revealed a plot to kidnap Demetracopoulos from the streets of Washington. The story was confirmed in 1979 by the junta's last ambassador to the United States, who related the details in his memoirs.

Based on our 1975 reports, two congressional committees that were investigating the intelligence community began examining U.S. relations with the fallen junta and the Demetracopoulos affair. According to investigative reporter Seymour Hersh, Kissinger personally intervened to kill the investigation. In Hersh's 1983 best-seller, ''The Price of Power,'' he wrote: ''Sources close to {the Senate Intelligence Committee} have said that its investigation was abruptly canceled at Kissinger's direct request. He urged the committee to drop the investigation, one official said, on the ground that relations between the United States and Greece could be 'severely harmed.' ''

In 1976 an undaunted Demetracopoulos enlisted the help of Washington attorney William Dobrovir and began bombarding the bureaucracy with requests for files based on the Freedom of Information and Privacy acts. He received hundreds of documents from the FBI, CIA and departments of Defense, Justice and State. Many of the papers indicated that copies had been provided to Kissinger's National Security Council, but all requests for NSC documents were rejected on the grounds that the documents were not in the NSC files. ''Except for those he holds, Mr. Kissinger's papers were remanded to the National Archives at the termination of the Nixon and Ford presidencies,'' one NSC letter said.

There was a catch. The papers that Kissinger had held -- thousands of pages of documents he considered ''personal papers'' -- had been copied for his own use and the originals had been deeded to the Library of Congress with the proviso that they not be made available to the public for 25 years.

In March 1977 the NSC released the only material it could dredge up -- computer indices of files that had existed on Demetracopoulos. Included was a cryptic reference to ''Mr. Demetracopoulos' death in an Athens prison.'' The date of the reference, Dec. 18, 1970, was about the same time that John Mitchell was proposing that Demetracopoulos be deported to Greece.

Since Kissinger had banned inspection of his papers at the Library of Congress, attorney Dobrovir asked for them directly from Kissinger. Kissinger refused for more than seven years to respond to Dobrovir's requests for copies of the files. Finally, last Nov. 30, Kissinger's attorney notified Dobrovir that ''efforts were made to search the collection {of Kissinger's personal papers} for copies of documents which meet the description provided. . . . No such copies could be found.''

Kissinger's attorney then suggested -- in so many words -- that Demetracopoulos spend the next couple of decades assembling the bits and pieces of his files from the numerous agencies that might have kept them. The originals, the attorney wrote, ''should be in the complete institutional files of the agencies which created and received them.''