The Fairfax County medical examiner, James C. Beyer, confirmed yesterday that it was the body of 12-year-old Billy Viscidi that was discovered Saturday afternoon in a shallow grave in the backyard of the Viscidi's Vienna home.

The identity of the badly decomposed body was established through dental records, Beyer said. The boy, who had been reported missing from his home at 503 Princeton Ter. nearly three weeks ago, was found clad in the same cut-off blue jeans he had been wearing when he was last seen.

Beyer said that Viscidi died of a massive fracture at the base of his skull. He refused to speculate on what might have caused such a fracture at the back of the 12-year-old's head.

Although Fairfax County police have not yet listed the death as a homicide, they have assigned homicide detectives to the case.

Police dug up the grave, located just at the edge of the Viscidi property at 2:30 p.m. Saturday, after the boy's mother, I. Grace Viscidi, called and asked them to again search the backyard.

Earlier Saturday, Mrs. Viscidi had called two family friends, coworkers of her husband Burton's, and asked them to search the area around the house "once more time," police said.

The grave had been dug in the garden behind a row of corn stalks, backed up against a row of bushes. Watts said, "it was very possible," that searchers could have stepped right over the grave because it had been well camouflaged.

Saturday, Mrs. Viscidi's friends noticed "some earth that didn't look right," according to neighbors and tried to put a shovel in the ground there. When the blade struck something, breaking the handle, they called police.

Homicide detective Squire Brooks said yesterday, "We haven't ruled anybody out yet.We are doing a complete check of anybody and everybody who knew anything about him. We have no serious suspects at this time."

Although Brooks refused to comment on any specifics of the investigation, police sources said that anyone who might have any information in the case - including neighbors, friends and family members - will be routinely questioned.

Aside from Billy's mother I. Grace, a computer programmer with the CIA, and his father Burton, a broadcast technician with Voice of America, he is survived by three brothers: Steve, 16, Larry, 14, and Kenney, 10.

The boys are visiting relatives in the New York City area, according to police and neighbors. Mrs. Viscidi returned to Georgetown University Hospital in Washington last night to spend the night in the room of her husband, who is recovering from minor surgery.

Burton Viscidi learned of the discovery of the body Saturday while watching the television news, according to a hospital spokesman. The Viscidi's lawyer, Douglas L. Pierson, refused to make any comment on the case last night except to say he had instructed all family members not to talk to the press.

Brooks said that the county police were planning to routinely search the Viscidi home, which had been searched earlier by Vienna police.

Vienna police Sgt. Joseph Tavares admitted that he had been shocked by the sight that greeted him when he arrived at the Viscidi's one-story brick rambler home on Saturday.

"I've seen dead adults," he said. "But seeing a missing child in a bag - that's too much. It's a hell of a lot harder to see a child because an adult has pretty much gone into adulthood and may have provoked something. A child - a 12-year-old - how could a child provoke something?"

That question was being asked throughout the quiet neighborhood of handsome one- and two-storey homes yesterday, as police steadfastly refused to discuss what they knew or did not know.

According to coworkers of Viscidi at Voice of America, the Viscidi's moved to this area from the New York City area, about 3 1/2 years ago. For a two-month period about two years ago Mrs. Viscidi worked at Voice of America library division using her first name - Ida. Since taking a job at the Central Intelligence Agency as a computer programmer she used the name I. Grace.

Mrs. Viscidi had made a tearful appeal on local television and radio stations Thursday, saying "if Billy is able to come home, we want him home."

But, police said, although an exact time of death has not been determined, they believe Viscidi had been dead for "at least several days, perhaps longer," when he was found.

Beyer said that further tests would be conducted on the body to try and determine an exact time of death - if possible - and whether the boy had been sexually molested.