The House Intelligence Committee has started an investigation of a CIA officer's snooping last year in the offices of another congressional committee.

Members of the Intelligence Committee, which was oversight authority over the CIA, were informed of the inquiry last week by Chairman Edward Boland (D-Mass.). Committee staffers had already interviewed the CIA's director of security, Robert Gambino, about the incident following a report in The Washington Post.

The Post, quoting informed sources, reported that the most sensitive files of the House Assassinations Committee had been rifled last summer and fingerprints on them traced to a CIA liason officer assigned to the committee.

The assignment, it has since been learned, was made under a CIA program code-named "MH/Child," which sources described as encompassing a variety of so-called "babysitting" chores.

The agency dismissed the liaison oofficer in question, Regis T. Blahut, last August and then dropped the matter. In a memo to all CIA employes last week, CIA Director Stansfield Turner took the position that Blahut had "acted alone and out of curiosity."

Since then, the CIA has also been insisting that Blahut, who had been employed by the agency's Office of Security, did not rifle the Assassinations Committee's files and did not even enter the safe where the files were kept.

However, the agency has refused to say what it thinks did happen, beyond describing it as - in the words of CIA spokesman Herbert Hetu - "something dumb."

According to informed sources, the incident took place one afternoon last July after an Assassinations Committee staffer had started inspecting some of the materials in a combination safe reserved for physical evidence of the 1963 assassination of President Kennedy.

Sources said he took what he wanted and left the room. According to one version, he left the safe door closed but unlocked; according to another, the safe door might have been left slightly ajar. In any case, sources said, when he returned, he found a book of Kennedy autopsy photos inside the safe in obvious disarray.

Blahut's fingerprints were found on the inside door or the safe. They were also found on one of the gruesome authopsy photos, which had been taken out of its plastic case, sources said. The plastic case itself had been torn from its notebook binder.

The CIA's Hetu, however, declares that Blahut "did not enter the safe to get the notebook."

Asked how it was then that Blahut's fingerprints were found on the inside of the safe door, Hetu said this must have happened when Blahut was "putting it [the book of photos] back in the safe."

Asked why Blahut would do that if he hadn't taken the book from the safe to begin with, the CIA spokesman said, "I don't know. Ask Blahut."

Hetu also declined to give the CIA's version of what happened, insisting that it would serve "no purpose."

Blahut, who reportedly failed CIA polygraph tests in several important respects concerning the incident, declines to comment. He has said there is an innocent explanation, but he has refused to say what that is.

Sources said there were at least three personnel shifts and changes within the CIA's Office of Security following the incident last summer and the recent publicity over it, affecting, among others, Blahut's immediate supervisor. Hetu said there was no connection.

"We're satisfied that what he [Blahut] did, he did on his own," the CIA spokesman told a reporter. "None of the things [personnel shifts and resignations] you've described had anything to do with that. . . We're satisfied the guy did something dumb. He looked at a book he wasn't supposed to look at. And we fired him after we assured ourselves that he wasn't tasked [to do with did] by anyone either inside or outside the agency."

Sources close to the committee maintained that the CIA's investigation was aimed primarily at getting the agency off the hook and was not thorough enough to eliminate suspicions that more than "curiosity" might have been involved.

Blahut had been assigned to help the Assassinations Committee with the CIA files it needed in its inquiries. Sources described the MH/Child project, under which the assignment came, as a sort of "babysitting" function that also includes escorting visitors to the CIA headquarters building at Langley.