Fairfax County Prosecutor Robert F. Horan Jr. said yesterday that Fairfax authorities, trying to unravel the mystery of how 12-year-old Billy Viscidi died, have not moved forward in the case because police have been unable to interview some Viscidi family members.

Horan said that the lack of movement in the investigation was "through no fault of our own."

"There are people in the family we would like to interview but we have been unable to do so," Horan said. That contention was denied yesterday by the Viscidi family lawyer.

Vienna police Sgt. Joseph Tavares said yesterday that Billy's father, Burton, had told him recently that 15-year-old Larry Viscidi, one of the two brothers at home with Billy when he disappeared on July 25, "may have suffered a blackout when Billy died or was killed."

Tavares said the older Viscidi told him the family planned psychiatric tests to see if such a blackout could have occurred.

But Horan said yesterday, "We don't think this is a plausible theory - because of certain things we know." He declined to say what those "certain things" were.

"You can always get some doctor to cite the amnesia theory," Horan said. "If there was amnesia in this case it was very selective amnesia."

Assistant Commonwealth's Attorney Steven A. Merril said yesterday that "we will meet with police tomorrow to see where we are." He gave no indication that any break in the case was imminent.

The case began when Billy was reported missing from the family's Vienna home and feared abducted or kidnaped. Police searches of nearby areas failed to find the youth, but on Aug. 12 family friends searching the property discovered his body in a shallow grave near the backyard garden.

An autopsy showed that Billy had died of a massive skull fracture, but police investigators have been unable to answer the tantalizing question: How did Billy die?

Horan said yesterday that authorities still don't know "beyond reasonable doubt" whether the fracture was caused accidentally or through foul play.

Viscidi family lawyer Douglas Pierson said yesterday that the family has "cooperated entirely with the commonwealth . . . They haven't been asked anything that they haven't given an answer to."

Horan said he gets phone calls and letters from around the nation asking him, in various ways, "Why are you protecting that murderer?"

But Horan said he has no answers not even any theories.

"We can't deal in speculation," he said."Newspapers, television and the public can deal in speculation. We have to prove what did occur."

"Here we have a dead boy who died from a skull fracture," Horan said. "That fracture is consistent with a blow, and it is also consistent with a fall, with some momentum behind it. The law does not allow you to draw the inference that the fracture was caused by a blow." The second inference - that there was a fall, possibly accidental - stands in the way.

"In every criminal trial, we must rule out every reasonable hypothesis of innocence. We must tilt all the evidence toward guilty - beyond a reasonable doubt."