There is "no way in the world" the mother of 12-year-old Billy Viscidi could have known that her son was buried in the back yard of her Vienna home before his body was discovered on Saturday, Fairfax County Commonwealth's Attorney Robert F. Horan Jr. said yesterday.

Horan also disclosed that there were other items found inside the plastic bag in which Billy was buried. He said those items "could have been Billy's clothing" or clothing belonging to someone else.

Evidence gathered in the investigation of Billy's death, which Horan says is surrounded by a "very bizarre set of circumstances," indicates that the boy's mother, I. Grace Viscidi, could not have known about Billy's death until Saturday, Horan said.

Continuing his reluctance to divulge many details of youth's death, the prosecutor would not say what evidence convinced him that Mrs. Viscidi was unaware before Saturday of the grave.

"Let me say this," Horan said. "If she knew he was out there in the backyard, she would have told everybody the minute she found out."

Mrs. Viscidi, after 18 days of worrying about Billy's disappearance on July 25, asked friends to look around her house for traces of the boy, Horan said.The body was found stuffed in a plastic bag in a shallow red clay grave.

Horan said Mrs. Viscidi asked her neighbors to search near the house because she was puzzled why Billy, who was first thought to be a runaway, would leave home without his shoes. The shoes that Billy normally wore were left in the house when he disappeared, Horan said.

When Billy's body was found he was clad in the cutoff jeans he was reported wearing when he disappeared, and he was barefoot. He was not wearing the red tank top shiret he was reported to have had on.

"The mother was always bothered by the fact that he didn't have his shoes on," Horan said. "It was something that seemed to her out of place. She wondered if he really did leave without those shoes.

Mrs. Viscidi was unavailable for comment yesterday on Horan's statements. She and two of her sons, who have cooperated this week in the Fairfax police investigation of the death, will not make a public statement about the case until "we can find the facts," according to family attorney Douglas L. Pierson.

Horan said he does not think it "necessary" to the investigation that county detectives interview the Viscidi's oldest son, Steve, 16, who at last report was visiting relatives in New York.

It is unlikely, Horan said, taht Steve will be interviewed before Billy's funeral this afternoon at the Money and King Funeral Home in Vienna.

Police, in a search warrant affidavit filed on Monday, said they believe that Billy died after being assaulted in the living room of his house. An autopsy showed the boy died after a powerful blow to back of the head, a blow Horan said was caused by a "blunt instrument."

The body was buried in the back yard grave within 24 hours of Billy's death, Horan has said. That conclussion is based on forensic evidence taken from the body and other still secret evidence gathered by investigators.

The prosecutor's office is awaiting the results of blood tests being conducted in Richmond on a piece of rug and chips from a bookcase from the Viscidi's home at 503 Princeton Ter. The items were taken from the home in a search on Sunday.

The blood tests have confirmed the presence of blood, but tests are still needed to determine if the blood is human and if it is Billy's type of blood. The tests are scheduled to be completed next week.

In the neighborhood of about 20 brick rambler houses where the Viscidis live, neighbors this week have expressed doubts about whether they should attend Billy's funeral.

Several neighbors said they fear the funeral will become "a media circus." One neighbor said she would stay away because she wanted to avoid a "mob scene" and all the "gawkers" who have been driving past the Viscidi house this week.

Neighbors said that Billy was well-like. One described the brown-haired boy as "quiet," another called him "lovable."

But many neighbors agreed tht Billy's parents, I. Grace and Burton, are not well-known in the area.

Charles E. Gray, who lives at 500 Princess Ct., said he often rode the commuter bus home from work with Burton Viscidi, a broadcast technician for the Voice of American, but that he never got to know him. "We just talked about the weather or the traffic," Gray said.

Another neighbor, who did not want her name used, said she didn't want to go the funeral because she did not know how Billy died. "You just wonder," the neighbor said.