The House Assassinations Committee discovered last summer that its most sensitive files had been rifled, and then traced fingerprints on them to an officer of the CIA, according to informed sources.

The incident involved surreptitious entry of a combination safe at the congressional committee's offices, the sources said. The safe was reserved for physical evidence of President Kennedy's assassination, including the autopsy photos, X-rays and other articles, such as the so-called "magic bullet" that wounded both Kennedy and Texas Gov. John B. Connally.

Apparently nothing had been taken, but, the source said, there was no doubt that the files in the safe had been tampered with. For instance, they said the autospy photos of the head shot that killed Kennedy had been taken out of their slip cases and were left in disarray inside the three-drawer safe.

"It looked as though someone had just run out," one source said.

After several inquiries by a reporter this week, the CIA acknowledged that it has dismissed the individual in question, but indicated that it plans no further action.

"We're satisfied that it was just a matter of curiosity [on the individual CIA officer's part]," said CIA spokesman Herbert Hetu.

Asked whether it might have been a matter of conscious CIA spying on a congressional committee, Hetu replied, "Good, lord, no."

The unauthorized entry was discovered when a committee staff member went to inspect some autopsy photos in the safe one afternoon, probably in July, sources said.

"Blakely [the House committee's chief counsel, G. Robert Blakey] was told right away," one source recounted. "Only three or four people were supposed to have access to that safe. And I understand that one of them said he'd locked it the night before."

Fingerprint experts from the D.C. plolice department, where several committee staffers had old friends, were called in. By then, someone had thoughtlessly had the documents rearranged neatly, so that there were other prints on them and on the safe. But the security-conscious committee reportedly had fingerprint records of everyone who worked for it, both those with access to the safe and those who had no business being there.

Sources said the only unauthorized set of prints the police found belonged to Regis . Blahut, a CIA liaison officer who had been detailed to assist the committee with the CIA records it needed for its investigations.

"His fingerprints were all over the place," one source reported. "On the photos, inside the safe, and on all sorts of different packages."

Particularly telling, another source indicated, was the fact that some of the prints were found on autopsy photos themselves rather than the plastic sleeves in which they had been encased.

The episode reportedly produced a great wave of anxiety within the CIA, which has been claiming for several years that it has learned its lessons and that its domestic spying and misdeeds are a relic of the past. In any case, the agency launched an intensive internal investigation, including polygraph examinations of Blahut and perhaps a number of his superiors.

In a brief telephone interview with The Washington Post, Blahut denied any wrongdoing. He acknowledged that his fingerprints had been found on the documents in question, but insisted that there was an innocent explanation. He refused, however, to say what that was.

"There's other things that are involved that are detrimental to other things." he said. Asked what he meant by that, he refused to elaborate.

"I signed an oath of secrecy [with the CIA]," he said. "I cannot discuss it any further."

Sources quoted Blakey, who was kept informed of the CIA's in-house inquiry, as having stated on several occasions that Blahut had been given three polygraph examinations in all and that he had failed them in important respects.

"He denied he did it, and he flunked that," one source said. "They also asked him whether anyone ordered him to do it. He said no one, and he flunked that."

Blahut, who said he worked for that CIA office of security, insisted that he had come through the tests with his credibility unblemished.

"I've alreadly defended myself to my employers," he said when he asked about the incident. "As far as I'm concerned, that's all cleared up."

Blakey, who has been working on the now moribund Assassination Committee's final report in recent weeks, refused to comment. Sources said he seized on the incident last year and used it as leverage to get the CIA to cough up a number of documents it had been holding back from the committee. Some of the records reportedly pertained to Lee Harvey Oswald's visit to Mexico City in September 1963.

"There was a marked improvement," one former staffer recalled. "All of a sudden, they were giving us everything we wanted. Blakey kept saying he wanted to go slow, to let them [the CIA] conduct the investigation . . . But I think he'd have to admit we wanted better cooperation."

Asked one question after another about the incident, including the identification of the CIA officer's fingerprints, Blakey kept saying: "I won't discuss the matter." Asked if he would deny it, he said, "No."

Most members of the House committee apparently were kept in the dark. Even the chairman of the subcommittee that investigated the Kennedy assassination, Rep. Richardson Preyer (D-N.C.), said he was unaware of it when queried by a reporter. Later, after checking with Chairman Louis Stokes (D-Ohio), Preyer declined to comment beyond saying:

"Blakey and Lou [Stokes] were handling the CIA stuff. I don't have my nose out of joint about it. Talk to Lou."

Stokes declined to talk. "The matter was terminated," he said. "There's no need for me to comment."

It was not clear what other CIA officials might have been given polygraph tests before the inquiry was dropped although sources said that one of Blahut's superiors, Scott Breckinridge of the CIA inspector general's office, had been expected to be given one. There were also reports that CIA Deputy Director Frank Carlucci had offered "in a magnanimous way" to take one.

Breckinridge is a veteran CIA official who served as the agency's chief liaison officer with the Senate Intelligence Committee during its 1975-56 investigations of the intelligence community. He also wrote the top-secret CIA inspector general's report in 1967 on CIA assassination plots against Cuban Premier Fidel Castro.

Breckinridge retired recently. He could not be reached for comment. CIA spokesman Hetu said his retirement had nothing to do with the rummaging of the House committee's safe.

As for Carlucci, Hutu told a reporter, "He doesn't remember having said what you said he said."