Billy Viscidi, the missing 12-year-old boy whose body was found Saturday buried in the back yard of his Vienna home, died after being assaulted in the living room of the house, Fairfax County police believe.

The Viscidi house became the focus of the investigation following discovery of a large stain, believed to be blood, on the living room rug and what appeared to be bloodstains on a nearby bookcase, according to an affidavit filed in the case. A police source also said that investigators now believe the grave in which Billy was found was only "four or five days old." The youth had been missing for three weeks before his body was found.

Fairfax County Commonwealth's Attorney Robert F. Horan Jr. said yesterday, however, that "there are no suspects at this point."

He also said that police attempts to question Billy's three brothers - Steve, 16, Larry, 14; and Kenny, 10 - were being frustrated by their mother's refusal to talk to them. The mother, I. Grace Viscide, made a tearful televised appeal last week for help in locating her then-missing son.

"There is no power on earth that can allow us to talk to the children if the parents say no." Horan said in an interview. The stains are being analyzed by a laboratory to see if they match Billy's type, Horan added.

An autopsy showed that Billy had died of a skull fracture caused by a poweful blow to the back of the head.

Police found the carpet and the bookcase - and took a 78- by-58 inch section of the carpet and chips from the bookcase - when they broke into the locked and unoccupied house, at 503 Princeton Terr., Vienna, early Sunday evening.

The few details of what the investigation has turned up appeared in the affidavit that Detective Phil Grimes filed with Magistrate Raymond E. Shaw to justify Sunday's search. The Viscidi house has been unoccupied since last week - when the children reportedly went to stay with relatives in New York and Mrs. Viscide went to Georgetown Hospital to be with her ailing husband.

In his affidavit, Grimes said "he verily believes that a carpet stained with blood is in the residence and that the carpet when analyzed will tend to show that (Billy) was assaulted while on that carpet and died as a result of that assault."

Grimes also said in the affidavit:

"The investigation turned up a concerned citizen who knows the mother. Concerned citizen states that I. Grace Viscide, the mother, told her that on July 25 she noticed a large spot on the living room carpet that was not there when she had gone to work the morning of July 25 (the day Billy was reported missing).

"She asked her son, Lawrence M., what the stain was and he replied that it was vomit where he had gotten sick that day. Mrs. Viscidi concerned citizen on Aug. 8 if she knew how she could get the stain analyzed without the police knowing about it. Mrs. Viscide state that she thought the stain was blood.

"On Aug. 12, a private investigator hired by Grace told another concerned citizen that . . . he had taken some of the carpet to be analyzed for blood . . . "%TLate yesterday afternoon, two private investigators hired by Mrs. Viscidi - Gordon Butler and M. Morgan Cherry of the Legal Investigations Inc., agency in Annadale - went to the house and left with some shirts and pants. The private detectives opened the door with a key.

In early evening, Butler and Cherry returned to the house but would not say why they were there,

Commonwealth's Attorney Horan said Larry and Kenny have returned from New York and are staying in Washington. Steve, the oldest, remained in New York, Horan said.

Mrs. Viscidi, a computer operator with the Central Intelligence Agency, has been staying at Georgetown Hospital, where her husband, Burton, a broadcast technician with the Voice of America, is recuperating from a minor surgery.

A highly placed Fairfax police official said the grave where Billy's body was found Saturday was fresh - not more than four of five days old. Two officers were required to lift the body out of the grave, the official said.

The official also said he theorized that the body may have been hidden in a crawl space under the house and then transferred to the two-foot-deep grave. But at the same time he said he didn't see how search dogs could have missed detecting the body. Soil samples were taken to determine if the body had been buried or hidden somewhere before it was buried in the garden, the official said.

The official was critical of the initial phase of the investigation, which was conducted by the Vienna town police before Fairfax authorities became involved about 10 days ago.

The official described the Vienna police work as "sloppy" and said Fairfax police were not called in soon enough. "A number of things that should have been covered (early in the investigation) were not," the official said.

Horan said the stain on the living room rug was not readily identifiable as blood. "The blood wasn't all that visible. A bloodstain does not look like a bloodstain after eight hours. Sometimes it looks like vomit," he said.

Horan also said that because the Viscidi boy's body was do decomposed, lab tests probably will not be able to determine when the body was put in the grave.

Neighborhood children who ordinarily play along Princeton Terrace and Princess Court were not there yesterday. Fearful parents said they have been keeping their children off the street since Billy Viscidi was reported missing 19 days ago.

"We are all scared to death," said Charles E. Gray, who lives at 500 Princess Ct. SW, the street behind the Viscidi home. "Everyone is running scared," he said.

Gray, who has been living in the neighborhood for 13 years, said the first time he saw Mrs. Viscidi, who moved to the area about four years ago, was when she appeared on television since Billy's disppearance.