A Fairfax County juvenile judge ruled yesterday that 15-year-old Larry Viscidi will face charges as a juvenile in connection with the death of his 12-year-old brother Billy and set Dec. 8 for the closed-door trial.

Judge Thomas A. Fortkort's decision followed testimony by a court probation officer and a Viscidi family psychiatrist that an investigation of the youth showed he was "amenable" to treatment as a juvenile, according to Vincent M. Picciano, director of juvenile court services.

Viscide, accompanied by his parents, sat quietly through the two hour and 15 minute hearing in a first-floor courtroom of the county courthouse. Reporters were barred from the proceeding.

If found "not innocent" of involuntary manslaughter, as he is charged, at next month's trial, the maximum penalty Viscidi could face is imprisonment until his 18th birthday, Picciano said, although the court could retain jurisdiction over him "within the community" until he is 21.

Commonwealth's Attorney Robert F. Horan Jr. said the youth, if convicted, would probably be released "inside a year" and would serve less than two years even if tried and convicted as an adult.

Yesterday's determination by the judge thus made "little or no difference" in the possible length of detention, Horan said.

"How do you establish he [Viscidi] is not amenable to treatment?" Horan asked reporters before the hearing began "With a juvenile who has no prior record there is no evidence that he is not amenable to treatment."

Later, Horan said that as soon as he had heard the report of the court probation officer urging that Viscidi be kept within the juvenile system, he told Fortkort he "didn't want it transferred" to the adult courts.

Similing slightly and staring straight ahead, Viscidi left the courthouse by a back exit, ignored a crowd of waiting reporters and was whisked away by his parents in the family's bronze Chevrolet.

After a court hearing last month, Viscidi left the courthouse wearing a brown paper grocery bag over his head to keep his pciture, his parents said, from being "splashed around" in the newspapers.

Since he was released from police custody on Sept. 18, Viscidi has been secluded under the court-approved supervision of his parents at their home in suburban Vienna - an arrangement that brought expressions of dismay from some troubled neighbors. He has not been attending school.

Piccisno said yesterday that that custody arrangements will continue until the trial next month.

Yesterday's decision means that the public is unlikely ever to learn details about how Billy Viscidi died or about events that led to discovery of his body in a shallow grave in the backyard of the Viscidi home.

The youth became the subject of a massive search by police and volunteers after he apparently walked away from the home on July 25. His mother, Grace, a computer programmer at the CIA, issued tearful appeals for his return, and his father, Burton, a technician at the Voice of America, left a Washington hospital where he was recovering from surgery to join in the search.

The boy's body was found Aug. 12 buried near a garden plot on the family property after his mother asked friends to renew their search.

Larry Viscidi was arrested on Sept. 13 and charged with murder in connection with the death, but the charge was reduced on Oct. 17 after Judge Fortkort ruled there was insufficient evidence to support it.

Horan, like other attorneys and court officials, has followed the confidentiality provisions of Virginia's juvenile law and never publically acknowledged that it is Larry Viscidi who is to face trial.

Yesterday Horan referred to "the juvenile charged in the Vienna murder case" - as Larry Viscidi also has been referred to in court.

The fact that Viscidi is to be tried within the juvenile court system means, the prosecutor's office has said in the past, that he will not be saddled with a permanent criminal record or lose his rights.

The "social investigation" presented in court yesterday is required by Virginia juvenile law and delves into a youth's family background, school activity and psychiatric history in determining whether he or she should be handled as a minor by the court.