IT CAME as a surprise to learn from Post reporter Don Oberdorfer's front-page story Jan. 17 that until a few weeks ago at least some officials of the Naval Investigative Service still believed that Marine guards at the U.S. Embassy in Moscow had opened the building to Soviet agents. After extensive interviews with Sgt. Clayton Lonetree in November, however, even these die-hards have conceded that the conspiracy never existed. The issue that remains unresolved, though, is how the wildly overblown charges were made and believed in the first place.

Sgt. Lonetree actually did have prohibited contacts with Soviet agents when he was stationed in Moscow in 1986. He has been court-martialed and found guilty of providing written and verbal information to them about embassy operations and personnel. But in an effort to determine the extent of the damage and examine the possibility that similar conduct had occurred at other embassies around the world, investigators clearly went off the track at an early stage of the inquiry and badly bungled the job. Innocent Marines were subjected to long and isolated interrogation and eventually signed statements -- immediately repudiated -- that had apparently been drafted by NIS agents. Cpl. Arnold Bracy was said to have conspired with Sgt. Lonetree in allowing Soviet agents entry into the embassy and access to secret documents and facilities. Two other Marines affirmed that Cpl. Bracy had confessed to them. As a result, the embassy in Moscow was crippled for months, unfair accusations were made against the U.S. ambassador, expensive equipment was shipped home and replaced and relations between the two superpowers were severely strained. Thirty million dollars was spent pursuing this investigation, and the reputations of a number of young men and of the Marine Corps itself were unjustly tarnished.

It is not enough to dismiss this whole episode as an example of unfortunate but perhaps necessary overreaction. Important questions remain, chief among them, as former assistant secretary of state Patt Derian pointed out on the opposite page this week: How did these flawed and explosive confessions come to be made in the first place? Is the NIS a competent investigative force? Were the circumstances of the interrogation fair or designed to elicit sensational statements whether true or not? Were threats made or physical force used? Who was in charge of checking out details of the "confession" -- it is now clear that they could not possibly have been true -- before action was taken?

The NIS has recently investigated itself and, not surprisingly, come up clean. But none of the errors has been explained, no accountability accepted. The Navy must assign responsibility for this mess and produce the facts.