The family of 12-year-old Billy Viscidi broke its silence yesterday and spent the day talking to Fairfax County law enforcement officials about events after the boy's reported disappearance July 27 and before the discovery of his body last Saturday in a shallow grave in his own back yard in Vienna.

Assistant prosecutor Steven A. Merril said he received "total cooperation" from the mother, I. Grace Viscidi, and two of Billy's brothers, Larry, 14, and Kenny, 10. But Merril would not comment on what the family members said, nor would he comment on progress in the investigation.

When asked by reporters about suspects in the case, Merril refused to rule out an accident.

"You don't have a suspect until you have a crime," he said. "You don't have a suspect when you have an accident, and we are still trying to determine whether we have a crime."

Billy's mother and two brothers were questioned by Merril at the Vienna office of the Viscidis' attorneys. The questioning, he said, occurred "off and on" throughout the day.

Fairfax County detectives returned yesterday to the viscidi home at 503 Princeton Terr. with the two Viscidi boys and Mrs. Viscidi.

During the afternoon Detective Phil Grimes removed from the house a four-foot-long object in a brown plastic bag, shaped like the base of a table lamp.

Grimes also removed a yellow plastic trash bag filled with soil taken from the dirt floor of a lawn shed in the Viscidis' back yard. Three metal trash cans also were removed.

The items were taken, Merril said yesterday, without a search warrant because of the family's cooperation in the investigation. Merril said Billy's third brother. Steve, 16, probably will be questioned later this week when he returns from New York where he has been visiting relatives.

After refusing to allow her children to cooperate in the investigation Monday, Mrs. Viscidi changed her mind yesterday morning and told her lawyer to contact Fairfax authorities, Merrill said.

According to Fairfax County prosecutor Robert F. Horan, parental permission is required because juveniles are subject "to the dominion and control of their parents." Juvenile courts in Fairfax have ruled consistently that in order to take a statement from a juvenile in a felony case, police must have the consent of the parents.

The police probe changed direction dramatically last weekend following discovery of the body, which Fairfax Commonwealth's Attorney Robert F. Horan said made earlier theories in the case "totally illogical."

"We are now looking at the close-to-the-home situation," Horan said.

"When we thought we were dealing with a runaway or kidnaping, police checked all the known child molesters in the area. But it is totally illogical to believe that someone kidnaped him and took him back to his own yard to bury him."

Horan also said, "There is much more focus on someone who might have some animosity toward him. We aren't dealing with a total stranger, a child molester. That possibility has been removed now."

One of the unusual elements in the case has been the role played by the private investigators, Gordon Butler and Morgan Cherry, of Legal Investigation Inc. of Annandale. They have had easy access to the Viscidi home and apparently to the family even when Fairfax authorities evidently did not.

The two said that they were at the family house yesterday as "glorifield babysitters" for Larry while Mrs. Viscidi met with Merril, police, and the Viscidis' two lawyers, Douglas L. Pierson and A. C. Soutzos.

In an interview, they said they do not know how or where Billy Viscidi died, but that it is their "gut feeling" that he died at the family's home.

They were off the case as of 5 p.m. yesterday, they said, having told Mrs. Viscidi that they had carried their investigation as far as they could.

Police on Monday said in "search warrant affidavit that they believe Billy, who was first considered a runaway or kidnap victim, died after being assaulted in the living room of his house.

On Sunday, police removed from the Viscidi home a piece of carpet and chips from a book case, which were believed to be stained with blood, according to the affidavit.

Horan said yesterday that the results of blood tests, which are being conducted at the Virginia state crime laboratory in Richmond, probably will be released tomorrow.

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 said that a "blunt instrument" caused the fracture.

Horan and Merril said yesterday that an auto accident in which Billy bumped his head about six weeks before his disappearance had no bearing on Billy's death. "The accident," Merril said, "has absolutely no significance."

Billy Viscidi was reported missing on Tuesday, July 25. His brother Larry told police he saw Billy leaving home and heading toward Nutley Street, a main thoroughfare in Vienna.

Vienna town police began searching for him immediately, and soon discounted the idea that Billy might have run away. His family said he was barefooted when he left.

The day after his disappearance, about 75 neighbors and members of two local amateur radio clubs began an intensive search centered in a two-mile square area near the Viscidi home near Rtes. 123 and Interstate 66.

Three days later, on July 29, Vienna town police and Fairfax County police called off the search, and two people said that a boy fitting Billy's description had been seen in a McDonald's restaurant near Fairfax Circle in Fairfax City accompanied by an older man. Investigators now say that may have been another child.

Both parents, I. Grace and Burton Viscidi, stayed home from their jobs after Billy's disappearance. Mrs. Viscidi is a computer programmer for the CIA and Viscidi is a broadcast technician for the Voice of America.

Burton Viscidi had been in the hospital for a kidney stone removal when Billy disappeared, but left to help look for his son. The Sunday after Billy's disappearance his brother Steve, 16 said: "We're really upset but we're trying not to think about it." His father described Billy as a "quiet boy" who liked to work with statistics. "He's very serious and hardly everlaughs," he said.

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

The next day Mr. Viscidi reentered Georgtown University Hospital for minor kidney surgery.

Three days after Mrs. Viscidi's televised appeal, Billy's body was found in a plastic bag buried at the edge of Viscidi's backyard. Police said the grave was not more than four of five days old.

By that time the other boys had been sent to visit relatives in New York, and the family's lawyer said he had instructed family members not to talk to investigators.

Also contributing to this story were Washington Post Staff Writers Diane Egner, Athelia Knight and Megan Rosenfeld.