A three-month long investigation of the Fairfax County Sheriffs Department has uncovered "a great number of criminal offenses" committed by several deputy sheriffs, but none that can be prosecuted, the county's prosecutor said yesterday.

Commonwealth's Attorney Robert F. Horan said he can't bring charges against the deputies because the offenses are misdemeanors committed more than a year ago. Under Virginia law, misdemeanors, crimes punishable by no more than 12 months in jail or fines of $1,000, must be prossecuted within a year of the offense.

The Fairfax investigation is continuing, but Horan said in an interview that it has failed to produce any evidence to support allegations that a county jail inmate built a fireplace at the home of Sheriff james D. Swinson.

Horan said he informed Swinson about the findings of his investigation three weeks ago and that it is up to Swinson to take some form of administrative action against the deputies involved. Several instances of "high-ranking" deputies involved. Several instances "high-ranking" deputies using jail inmates to perform work outside the jail were discovered in the investigation, the prosecutor said.

There were other instances in which deputies allowed inmates to leave the jail and to visit women. Horan said. In the fall of 1976 one deputy drove an inmate to a house in Alexandria and waited in a downstairs room while the inmate went to an upstairs room with a woman, he said.

Swinson said yesterday he has not yet decided what action, if any, he will take against the "three or four" deputies, according to his count, who were involved in the misuse of inmates.

"I don't expect the deputies to be little saints at all times, because I'm a realist," the sheriff said.

"I'll do what I have to do when I get to it," Swinson said. "Have you ever heard," he asked, "that where legal action fails administrative action is never a cure?"

The sheriff said he is planning to review each case individually. "I am not a man who makers snap judgements," he said.

Swinson, a Republican in his fourth term, said he and his department have taken considerable criticism in the past three months because of allegations concerning his fireplace and a cyclone fence built near his $100,000 home near Great Falls. "I have never been mixed up in anything illegal. Horan knows that and I know that. Every damn stick that went into my house I paid for," Swinson said.

Horan, a Democrat, supported Swinson's claims yesterday, saying that his investigation found no evidence that an inmate, who was alleged to be an expert stonemason, built the fireplace in the Swinson home. "The fireplace was in the house when Swinson moved in," Horan said.

Horan also said that his investigation has found no illegalities surrounding the purchase of the Cyclone fence. "It is a gift of a deputy who says that fence is scrap from Fort Belvoir (an Army post in Fairfax County). At Fort Belvoir they don't know where the fence came from," Horan said.

Swinson said that Horan's findings demonstrate that the sheriff's office has "integrity and it has always had that."

Horan said his investigation of the sheriffs department which he described as "difficult" because there is "an awful lot of innnendo" among the deputies, will continue until detectives have followed up all leads and "gripes."

The prosecutor said one of the reasons he is pursuning the inquiry is because a number of sources in the sheriff's department have told investigation that Swinson is not taking action against deputies who misused jail inmates because the sheriff fears the deputies may voice new public criticisms of him if they are reprimanded.

"If one of my people committed a crime. I would fire him," Horan said.

Swinson said that none of his deputies "have anything on him" and said he will make a statement when he is ready to announce any action against anyone in his department.

Swinson recently hired a new chief deputy to replace Myron L. Greenquist, whom Swinson fired in February "because we could not work as a team." The new chief deputy, M. Wayne Huggins, a 29-year-old former Virginia state trooper, has been introduced in Fairfax County in recent months as a possible successor to Swinson.