For almost three weeks, Sgt. Joe Tavares, a 16-year veteran of the Vienna police force, lived day and night with perhaps the most baffling and notorious case the department has handled - the mysterious disappearance of 12-year-old Billy Viscidi.

As head of the investigation, which drew on all the town's three detectives, the soft-spoken. 40ish Tavares chased scores of fruitless leads, listened to pyshics give their elaborate interpretations, and wondered, when an abduction was suspected, if another mother would be coming by the station to report her child missing.

Last Saturday, in a moment that even now he describes in measured, somber tones, Tavares saw Billy's decomposed body unearthed from its shallow grave behind the Viscidi home - a discovery that abruptly shifted the investigation's direction away from Tavares' earlier course.

On Tuesday, a still-shaken Tavares read a news story in which a high Fairfax County police official criticized the Vienna's force's early investigative efforts as "sloppy." "A number of things that should have been covered (early in the investigation) were not," the official said in the story.

"I felt as if I had been hit between the eyes," Tavares said in an interview. He rebutted all the criticisms, and said, "I know in my own conscience that we did everything we could. I don't think we deserve this kind of criticism."

Tired and hurt, Tavares yesterday took his first real holiday since the investigation began. With his wife, Sissy, and two children, Tavares jumped into his newly acquired second-hand camper truck and headed down the road to Busch Gardens near Williamsburg.

Tough how Billy died is still a mystery, for Tavares the investigation ended on the day the boy's body was discovered. That is when the investigation was turned over to the Fairfax police department.

Tavares disputes criticism that the Fairfax police were not called in early enough. He said the county police canine corps checked the neighborhood the evening Billy was reported missing. Other help, he said, came two days later when, after reports that Billy was seen in the Jermantown Road area near Fairfax City. Fairfax detectives stopped motorists on nearby roads and showed them pictures of Billy.

Three days after that, Tavares said, two Fairfax detectives spent their day off helping Vienna police fund a potential suspect.

As days went by with no leads on Billy's whereabouts, the story attracted increasing attention in the press and on television and radio.That attention, Tavares said, seemed to multiply the number of calls from people who said they had seen Billy or had some suggestions on how he could be found.

"The detectives were getting swamped," Tavares said. "There was no way we could handle the situation."

On July 31, Tavares said he got a call from the Fairfax criminal investigation department saying that it had been instructed by Police Chief Col. Richard A. King "to give them (the Vienna police) whatever assistance they wanted."

That day, Tavares said, six days after Billy disappeared, "We had a meeting with the town manager and town chief of police, and I said we needed more men. We had only four at that point - three detectives and myself. I asked for five more, and that's what I got, plus three detectives from the county."

Did the full-time help from the county come soon enough? Tavares answers ambiguously: "I don't see us doing anything differently. What we did was right, but with hindsight, we can say we probably needed help right from the beginning."

The stain on the Viscidis' living room carpet, which Fairfax police believe came from Billy's blood was not detected by Vienna police during their early searchers of the house. "It was faint, you could hardly see it," Tavares said - an opinion backed by Fairfax officials.

As to why the searchers did not detect the makeshift grave at the back corner of the Viscidi yard sooner than last Saturday, Tavares said the emphasis, up to that time, had been on the possibility of abduction.

Tavares is backed up by Fairfax Commonwealth's Attorney Robert F. Horan Jr., who yesterday called criticism of the Vienna force "the ultimate cheap shot. There's not a police force in the world that would have looked in the backyard. The last thing anybody would have done is look for a runaway or kidnapped child there" Horan said.

Summing up his feelings, Tavares said: "Sure, we made mistakes. But I don't feel we're incompetent. The impact of the (criticism) is dangerous. We still have to work in this community."

Solidly middle-class Vienna is probably the most crime-free community in urbanized Northern Virginia. The rate of crimes involving violence or property is lower in the town than it is in Fairfax or even in Fairfax City, which is demographically similar to Vienna.

As a result, Tavares says, the town police deal mostly with the routine and unremarkable: traffic infractions, burglary, grand larceny and vandalism.

But Steven A. Merril, Horan's chief deputy, says the Vienna police have been "competent" in investigations of crimes he has prosecuted. "I think the Vienna police have been given a very bad rap in this case," Merril said.