A Fairfax County juvenile court judge yesterday ended the celebrated Billy Viscidi case, dismissing an involuntary manslaughter charge against 15-year-old Larry Viscidi in the mysterious death of his brother last July.

Judge Arnold B. Kassabian ruled after a closed, 5 1/2 hour trial at Fairfax Courthouse that authorities had failed to muster sufficient evidence to show beyond a reasonable doubt that "the accused killed Billy Viscidi.

"To hold that the cause of death was the criminal act of the accused would amount to speculations and surmise," Kassabian said in a statement issued minutes after the end of yesterday's proceeding.

Because of the confidentiality provisions of Virginia's juvenile law, Larry Viscidi has never been referred to by name in court proceedings and officials have released few details surrounding the death.

The judge's decision appears to cap a sensational case that began last July 25 when 12-year-old Billy Viscidi was reported missing from his suburban Vienna home. Larry Viscidi was arrested Sept. 13 and charged with his brother's death after neighbors discovered Billy Viscidi's body buried in the backyard of the family's home.

In his statement, Kassabian said the prosecution had relied on an inference of guilt arising from Larry Viscidi's alleged burying of the body, which "standing alone is insufficient to prove the accused criminally responsible."

Yesterday, smiling widely, Larry Viscidi emerged from the first-floor courtroom accompanied by his parents and several people identified by a court official as family friends, and was promptly whisked away in the Viscidi's waiting automobile.

"Yeah, you..." was the diminutive youth's only response when one reporter asked if he was pleased with the verdict. Reporters had been barred from the proceeding. His reply was cut short by his mother, who ushered him toward the door.

Like their son, Burton Viscidi and I. Grace Viscidi ignored waiting reporters' questions in their hurried departure through a back door of the court-house.

"This is it. The case is ended," Commonwealth's Attorney Robert F. Horan Jr. said yesterday in a brief exchange with reporters, in which he declined to elaborate on the judge's decision.

But earlier, during a court recess, Horan had said that the mass of circumstantial evidence involved in the trial made the case "more complicated than most.

"We have no direct evidence, no videotapes we can replay," Horan said.

Yesterday's trial pitted Horan, who has prosecuted a number of sensational murder trials in his 12-year career as commonwealth's attorney, against a veteran Northern Virginia criminal lawyer, Louis Koutoulakos.

"The case was dismissed. How happy can you get?" asked Koutoulakos as he left the courthouse yesterday, beaming, but like Horan, refusing to answer reporters' questions.

Had Horan won a guilty verdict, the maximum penalty Larry Viscidi would have faced was confinement in a state institution until his 18th birth-day and probation until he turned 21, Horan said yesterday.

In the four months since Larry Viscidi was charged with his brother's death, both prosecution and detense have carefully refrained from discussing in public the facts surrounding Billy's death. Horan has said, however, that Billy Viscidi died of a blow to the back of his head that caused "a horrendous fracture."

Judge Kassabian's statement yesterday, citing medical testimony in the case, said the fracture might have been caused by "a running fall against an object."

That agreed, the judge said, with a statement given by Larry Viscidi to police that while downstairs at the family home at 503 Princeton Ter., he heard "a running, thud, bang, and yell, and went upstairs to find Billy lying on the floor near the bookcase."

Horan has said that Billy Viscidi's injury came from a "blunt instrument" and the judge's statement yesterday referred to an elephant-shaped statute.

But Kassabian said: "The court found it was just as, or more consistent that, the elephant fell off the shelf and landed next to the body, and thus was covered with blood, as it was that the accused killed Billy with it."

Billy Viscidi was first believed by police to be a kidnap victim or a runaway when his mother reported him missing on July 25. Larry Viscidi told a reporter at the time that his brother had eaten breakfast that morning, then "just walked down the driveway, took a left and headed toward Nutley (Road)," a main street in Vienna.

Grace Viscidi, a computer programmer at the CIA, issued a tearful televised appeal for her son's return, while Billy's father, Burton, a Voice of America technician, left a Washington hospital where he was recuperating from surgery to join the search.

A massive search by police and volunteers ended Aug. 12 when the boy's body was discovered by family friends on the Viscidi property wrapped in a yellow plastic bag.

Larry Viscidi was first charged with murder in his brother's death, but that charge was reduced to involuntary manslaughter Oct. 17 by another juvenile court judge, Thomas A. Fortkort, who said there was insufficient evidence to support a murder charge.

A court decision on Nov. 17 that the 15-year-old youth would be tried behind closed doors as a juvenile rather than as an adult, virtually guaranteed that the public would never learn details of how Billy Viscidi died or what events led to the discovery of his body.