Fairfax County prosecutor Robert F. Horan yesterday characterized the death of 12-year-old Billy Viscidi as "surrounded by an unnatural and very bizarre set of circumstances" and said investigators remain uncertain of how the Vienna boy died.

New evidence indicates that Billy was buried in a shallow red clay grave in his backyard within 24 hours of his disappearance, Horan said. During that period of time police and neighbors had begun to mount massive searches for him in the area.

Separately, in Richmond, a state crime laboratory forensic serologist indicated that tests have confirmed the presence of blood on a piece of carpet taken from the Viscidi home. Additional tests will be needed to confirm if that blood is human and is Billy's type of blood, she said.

But Horan and police remained silent on most details in their investigation, including what they have been told by members of the Viscidi family.

"If I could give you an honest answer about whether the death was accidental or murder, I would do it, I swear," said Deputy Commonwealth's Attorney Steven A. Merril, who questioned Billy's mother and two of his brothers Tuesday.

Commonwealth's Attorney Horan, describing the investigation as still in "the fact-finding stage," said Billy's oldest brother, Steve, 16, has not been interviewed by county investigators. "There is no great urgency about talking to him," Horan said.

The prosecutor said he expects some "preliminary findings" today on the blood discovered on the piece of carpet and chips from a book case. The items were taken from the Viscidi home in a police search Sunday.

In a search warrant affidavit filed Monday, police said they believe Billy, who was first considered a runway, or kidnap victim when he disappeared July 25, died after being assaulted in the living room of his house.

"To begin with a missing child and wind up with a missing child in the backyard is bizarre to say the least," Horan said yesterday.

The discovery of Billy's body Saturday has forced police investigators to begin reinterviewing neighbors and friends of the family in an effort to find out "if someone wanted to kill Billy," Horan said.

"We are looking at that neighborhood. The odds that a total stranger is involved are just out as far as I'm concerned," Horan said. He said five investigators are assigned to the case full-time.

Horan said the determination that the body was buried 24 hours after Billy died was based on forensic evidence taken from the body and on other "evidence." He would not comment on whether the questioning of the Viscidi boys contributed to that evidence.

"We are beginning to piece together what happened," Merril said yesterday. "We know a great deal more now then when the body was orginally found."

As part of the inquiry into Billy's death, county police investigators yesterday met with two private investigators hired by Billy's mother to help locate her son. Gordon Butler and Morgan Cherry of Legal Investigations Inc. of Annandale said they were with police four hours yesterday afternoon and that they each gave statements about their role in trying to find Billy.

An autopsy over the weekend showed that Billy had died of a skull fracture caused by a powerful blow to the back of the head. Horan says a "blunt instrument" caused the fracture.

The investigation, Horan said, has been unable to determine whether Billy was hit with the blunt instrument or whether his head accidentally hit that instrument.

A parade of reporters and cameras crews has been clogging the halls of the Fairfax County Courthouse, where Horan's office is located, seeking details of the case. Horan said the unusual nature of the death justifies the publicity, but he questioned the propriety of the media's attention toward the Viscidi family.

"I am surprised at the intensity of the media's coverage of the family. There are reporters staked out there at the house. The fact is this family is burying a son. It is a week of mourning for them," Horan said.

Funeral services for Billy are scheduled for 1:30 p.m. tomorrow at the Money and King Vienna Funeral Home, 171 W. Maple Ave. in Vienna. The body will be buried at Fairfax Memorial Park.

Parents have requested that any donations go to the Billy Viscide Memorial Fund in care of Glory Cromwell, of the pediatrics department at Georgetown Medical Center. The money will be held until Billy's donations go to the Billy Viscidi decide what they would like it to be spent on, according to hospital spokesman Cynthia Byers.

Ten days after Billy disappeared, Mrs. Viscidi made an emotional appeal on television for help. "I want my boy. If Billy is able to come home, we want him home," she said.

Mrs. Viscidi yesterday refused to talk with reporters. Her two sons, Larry, 14, and Kenny, 10, remained secluded in the family home yesterday with a family friend.

Horan has said that if police find Billy's death was accidental there is no Virginia law under which criminal charges can be brought against the parties involved.