A Fairfax County juvenile judge yesterday ordered Larry Viscidi, 15, held in police custody without bail in connection with the death of his brother, 12-year-old Billy Viscidi, last July.

Larry Viscidi was led, with his hands cuffed behind his back, from the Fairfax County courthouse following a closed, 28-minute detention hearing presided over by Judge Thomas Fortkort and attended by Viscidi's parents, Burton and I. Grace Viscidi, and the family's lawyer, Douglas L. Pierson.

Because of the confidentiality provisions of Virginia juvenile law, Commonwealth's Attorney Robert F. Horan Jr. refused to confirm yesterday that the juvenile arrested Wednesday night and charged with murder was Larry Viscidi.

The elder Viscidi, who sat in courthouse hallways for nearly two hours before yesterday's hearing began, appeared calm and refused to speak with reporters.

Vincent M. Picciano, director of juvenile court services, said after the hearing that "the juvenile" would remain in custody at the Northern Virginia Juvenile Detention Center in Lincolnia "for the time being."

Horan said that a preliminary hearing is set for Oct. 10.

Horan, who yesterday questioned the wisdom of the law that prohibits him from releasing an accused juvenile's name even in highly publicized cases, would say only that police finished their investigation Wednesday and that "our analysis" led to the arrest.

Police questioned two of the three Viscidi children on Monday, Horan said, but he would not say which two.

Billy Viscidi died on July 25 from a blow to the back of the head, which Horan has said caused a "horrendous fracture." The boy was assaulted with an unidentified "blunt instrument" in the living room of his house and died there within a few minutes near a spot where the carpet was stained with blood. Horan said.

The body was then placed inside a yellow plastic bag along with some "child's clothing which did not fit Billy," Horan said. The bag was buried in a shallow red clay grave in the back-yard garden.

On the day of Billy's disappearance the boy was reported missing by his mother, a computer programmer for the CIA. The next day, Larry told a reporter he had last seen his brother when Billy "just walked down the driveway, took a left and headed for Nutley (Road)," a major street in Vienna.

Vienna police theorized that Billy had run away or been abducted until Aug. 12 when friends of the family, at the urging of Billy's mother, searched the back yard and found the body.

Three days before the body was found, Mrs. Viscidi made a tearful televised appeal for the return of her son. "I want my boy," she said."If Billy is able to come home, we want him home."

Billy's father, who had been in Georgetown University Hospital recovering from surgery when the boy disappeared, left his bed to help look for his son. He returned to the hospital with medical complications two days before the body was found.

Two weeks ago Burton Viscidi told a Vienna policeman that Larry could not remember events that occurred on the day of Billy's death. On one occasion, Horan remarked that Billy's death was "surrounded by an unnatural and bizarre set of circumstances."

In the Viscidi neighborhood in southwest Vienna, where some residents had expressed concern in the past week that there had been no breakthrough in the investigation, there were expressions of relief yesterday that, finally, a suspect had been arrested.

"This has been a nightmare for all of us," said one neighbor, whose 10-year-old Kenney Viscidi, one of Billy's brothers, before the death.

"I feel relieved," said the woman, who asked not to be identified. "I also feel badly. It must be hard for the family, but it's better than going on the way it had been."

Some residents of the Townes of Moorefield town house cluster a block from the Viscidi home, on Princeton Terrace SW, had circulated a petition in the neighborhood that said they felt "insecure" and asked what was being done to solve the case. The signatures were to be presented to the Fairfax commonwealth's attorney's office.

Most of the concern was expressed by residents who have young children. "It's been very difficult around here," said one resident, who also asked not to be identified."I have not let my children go around the corner to Princeton Terrace. Now that there's been an arrest, I'm relieved - I hope so."

Diane Heinen, who lives in Townes of Moorefield, said "I've been concerned since the Lyon sisters disappeared. But you can't put your children in a box and not let them out."

Heinen's daughter, Debbie, said the arrest was not a topic of conversation at Oakton High School, where both she and Larry Viscidi are students. At school, she said, Larry "seemed more quiet than ever . . . He kept to himself."

Gloria Martin, one of the town house residents who signed the petition, summed up the reaction of many neighbors who were interviewed: "I feel better, and I think the Viscidi family should feel better too."

Prosecutors have not decided whether to seek to have the defendant tried as an adult in an open court proceeding, Deputy Commonwealth's Attorney Steven A. Merril said yesterday.

If the case is heard in a closed juvenile court, Merril said, the public will probably never know details of how Billy Viscidi died or who buried the boy.

In a Juvenile Court hearing, Horan said, murder charges against the youth could either be dismissed or a judge could find him "not innocent."

Such a finding would probably place the juvenile in the court's custody for treatment in a juvenile center until he reaches age 21. "That finding carries none of the ramifications of a guilty verdict," Merrill said. He said the youth would not have a permanent criminal record or lose his rights.

For "the juvenile" to be tried as an adult, Merril said the prosecution must request certification five days before the preliminary hearing. Then, the Juvenile Court must conduct a study of the youth's psychological and sociological background. This is presented, with defense and prosecution arguments, at a so-called certification hearing.

Merril said it is "difficult" for Virginia prosecutors to get any juvenile, regardless of his offense, certified as an adult.

For a juvenile to be certified as adult in Virginia he must be 15 years old at the time the crime was committed. It could not be learned yesterday whether Larry Viscidi had turned 15 at the time of his brother's death.

Horan said he would prefer that the details of the Viscidi case could be made public, but added that "the law doesn't say in those cases where the press expresses a great deal of interest that you can deal with it differently."