Investigators probing the death of 12-year-old Billy Viscidi have found out who buried the youth's body in the back yard of his Vienna home, Commonwealth's Attorney Robert F. Horan Jr. said yesterday.

Horan Jr. said yesterday.

Horan refused to divulge the name, obtained in interviews conducted last week, however, saying only that "legally you cannot draw the inference from who buried Billy to who killed him."

Police and prosecutors last week repeatedly questioned three members of Billy's family - his mother, I. Grace Viscidi, and two of his brothers, Larry, 14, and Kenny, 10 - concerning events that occurred between Billy's disappearance on July 25 and the discovery of his body on Aug. 12 by friends of the family.

Horan has said there is "no way in the world" that Billy's mother could have known about her son's death before the body was found. Burton Viscidi, Billy's father, has told reporters that both Larry and Kenny were at home all day on the day police say Billy died.

Horan also said yesterday that Billy died of a "horrendous fracture" to the back of the skull. The fracture, he said, could only have been caused by a blow of considerable force. "This was not a simple fall-down-on-the-rug fracture," he said.

Police have not been able to find what caused the fracture, Horan added, but said the boy could have been hit with a large, heavy, blunt instrument or his head could have been propelled" into a wall or a desk.

Billy probably sustained the blow to his head in the living room of his house and died there within a few minutes, Horan said, near a spot in the Viscidis' living room where the carpet was stained with blood.

The "normal" reaction to Billy's death, if there were no culpability, should have been for a witness to call somebody, Horan said.

Billy's death has not yet been classified as either an accident or murder.

"If it is a purely accidental death." Horan said, =the $64 question in this case is why wasn't the rescue squad called? If a death occurred in horse play among kids, why didn't someone call for help?"

Horan, who has disclosed details of the case sparingly since the body's discovery in a shallow grave, also revealed yesterday that Billy was buried in a yellow plastic bag that Horan said had once been used to store lawn mulch.

The body, which weighed about 65 pounds, was placed inside the bag along with some "child's clothing which did not fit Billy," Horan said. That clothing, which was bloodstained, is now being tested in Richmond, he said.

He added that the bag containing Billy's body could have been carried to the back yard garden, where it was buried, by "anybody strong enough to handle 65 pounds."

Investigators have recovered the shovel that was used to dig Billy's 30-inch-deep grave, Horan also said. He said the grave, which was dug in soft dirt that had been recently "turned over," took "some time" to dig.

Horan said medical examiners have been able to determine from the absence of fly eggs on the body that Billy was buried shortly after death.

Burying a body is not a crime in Virginia, according to Horan. He said that although the average person may think that whoever buried the body probably killed Billy, that he, as a prosecutor bound by the law, could not yet draw that conclusion.

Horan said yesterday that he cannot offer a complete explanation of Billy's death because he doesn't have the facts to justify any explanation.

The prosecutor said he and his family, at dinner parties in the past week, have encountered continuous speculation over "who killed Billy." Horan said he has his theories in the case, but as a prosecutor he must remain silent until all the facts are in from the police investigation.

"I don't have the luxury of speculation," the prosecutor said.

Billy, who has been described as a quiet, shy and intelligent boy, was buried a week ago after a simple funeral conducted by a Unitarian minister who had been chosen by Billy's mother the day before.

The Rev. Frank Robertson, a pastor at All Souls Unitarian Church in the District of Columbia, said after the furneral that Mrs. Viscidi, who is a comer programmer for the Central Intelligence Agency, told him she thought Billys death "was probably an accident."

Billy's father, Burton, who is a broadcast technician, remained yesterday in Georgetown University Hospital where he has been treated over the past two weeks for a kidney ailment.

When Billy disappeared and was first considered either a runaway or a Kidnaping victim, his father left the hospital to help search for the boy. He was hospitalized again two days before the body was found.

Three days before the body was found, Mrs. Viscidi made an emotional appeal on television for help. "I want my boy." she said. "If Billy is able to come home, we want him home."

Police obtained a search warrant and broke into the temporarily vacnt Viscidi home at 503 Princeton Ter the day after the body was found. They took the blood-stained piece of carpet and a piece of a hookcase smattered with what appeared to be blood.

Tests conducted in Richmond on a piece of the carpet revealed a human blood stain. Horan said, but there was not enough blood to allow conclusive tests on its type.

In an affidavit filed by the police with the searcg warrant, police said that Larry Viscidi had told his mother that the stain on the living room carpet, where tests had shown the presence of human blood, was vomit.

Police also said in the affidavit that they believed Billy died in the living room where he was assaulted.