The Senate Intelligence Committee has informed the widow of former high-level CIA official John A. Paisley that a lengthy investigation into his September 1978 death on the Chesapeake Bay has uncovered "no information which would detract from (his) record of outstanding performance in faithful service to his country."

The committee's full report -- and its reportedly detailed examination of the critical question of whether Paisley was murdered or committed suicide -- will remain secret, Committee Chairman Birch Bayh (D-Ind.) and Vice Chairman Barry Goldwater (R-Ariz.) said in a letter.

The FBI concluded in a previously secret report last year that there was "no credible evidence" to show that Paisley's death was "other than a suicide."

However, to many persons familiar with the case, that secret report and the Senate panel's letter to Mrs. Paisley are merely chapters in a real-life mystery that may never be solved to everyone's satisfaction.

Paisley, who as deputy director of the CIA's office of stragetic research analyzed some of the country's most sensitive nuclear stragetic capability data, retired from his full-time job at the agency in 1974 but went to work for it immediately as a part-time consultant.

He was on his sailboat on Chesapeake Bay in late September 1978 and working on an agency report when he failed to return to port at day's end as promised. His boat was found aground the next day, and he was missing. A week later, a badly decomposed body later identified as his was found in the bay with a gunshot wound in the head.

Conspiracy buffs have had a field day with the circumstances of Paisley's death. They have theorized that Paisley was a spy or a counterspy, or even that he was the Russian intelligence "mole" who many believe had infiltrated high levels of the CIA. Depending on which theory people espoused and whether they accepted the official version that the badly decomposed body later found in the Bay was Paisley's, the ex-CIA official could have been murdered or kidnaped or merely disappeared.

The various official investigations, though, discount any of those theories and accept what those probers feel is the most plausible view: that the Russian-speaking Paisley, who had been estranged from his wife and was undergoing psychiatric treatment, committed suicide by shooting himself in the head and falling into the bay after weighing himself down with two diver belts weighing a total of 38 pounds.

But, said a CIA spokesman who was clearly exasperated with continued questions about Paisley's death, "Nobody knows how it happened. We don't know how John Paisley died."

The CIA vehemently denies that Paisley ever was involved in clandestine affairs and has said earlier that it had "no reason to believe that John Paisley was other than he seemed, a highly respected former CIA analyst and manager . . ."

A CIA spokesman acknowledged yesterday that the agency added to the mystery when it played down the importance of Paisley's analyst job there when he was first reported missing. "In hindsight, we wish we had given out more [details about Paisley's career] at first," said CIA spokesman Dale Peterson.

He said it is the nature of the agency to be "very reluctant" to give out information about current or former employes and at the time it saw no reason to change that policy.

One intelligence community member not with the CIA supported that theory by saying the media "shouldn't discount as crazy the fact the CIA might have misled the public early just out of habit . . ."

Last March the FBI prepared a 39-page secret report at the request of the Senate intelligence Committee to examine various publicized theories about Paisley's death.

The report, portions of which have been made public, conclusively finds that the body recovered was Paisley's. "While the delay in fingerprint identification is regrettable, no intentional, conspiratorial or otherwise questionable connotations are attached thereto," the report said.

Virtually none of the detailed answers to questions about Paisley's alleged counterintelligence involvement were made public in the report.

One section of the report did conclude that "there is no basis in fact to allegations that, on the night of Paisley's disappearance, a Soviet vessel was proceeding up Chesapeake Bay or an unusual amount of communcations traffic emanated from the Soviet summer residence on the eastern shore of Chesapeake Bay."

The report also discloses for the first time that the FBI investigated a claim by a terrorist group in anonymous telephone calls that Paisley had been kidnapped, but concluded the calls were part of a hoax.

As to whether Paisley committed suicide or was killed, the report concludes, "This issue cannot be resolved conclusively." Instead, "It is concluded that no credible evidence exists to suspect John Arthur Paisley's death was other than a suicide."

For that reason, the FBI told the Senate committee, there were "no specific and articulable facts" upon which the base a federal investigation.

The committee's letter to Mrs. Paisley said that it had not conducted either a homicide investigation or a full counterintelligence investigation. The letters said the investigation found "no information [that the death] was connected in any way to involvement in foreign intelligence or counterinteligence matters."

It said it investigated "several other collateral intelligence issues" during the probe, but none of them had any "bearing on Mr. Paisley, either on his death, his loyalty or his career."

Mrs. Paisley said yesterday she was "disappointed the Senate committee classified the information [since] I had counted heavily on the committee to look into it thoroughly. I just would like to know the answers."

Mrs. Paisley's attorney, Bernard Fensterwald Jr., said he does not see "why they're so hush-hush about it" if Paisley was not involved in the secret activities.

Fensterwald, who is pressing Freedom of Information Act suits against the FBI and the CIA over the matter, said "one of the great disservices Congress did in passing the FOI act was to exempt themselves from it. I think Mrs. Paisley's entitled to know what happened to her husband."

The Maryland State Police, the only agency conducting a full homicide investigation of Paisley's death, said the official ruling so far is that the manner of death is "undetermined." A police official said the investigation was neither open or closed, but was considered "suspended."