Bellida Crisp was lying on a sundeck sofa four Saturdays ago, absently gazing at a trailer parker near her Dale city split-kevek house, oblivious to the execution-style murders taking place 150 feet away.

She lay there in the sun most of the afternoon. Her husband was cutting lumber nearby with a power saw and Mrs. Crisp heard nothing but the sound of the saw. She noticed people around inside the trailer which served as a sales office for a home building company, and, when police cars converged on the trailer around 7 p.m., she said she feared somebody must have had a heart attack.

What had happened was a triple murder the county prosecutor calls the "most heinous" crime in the history of Prime William County, a crime that he says "may be unsolvable." It is a crime that Bellida Crisp says has left her family and her neighbors with "a feeling of death that just doesn't go away."

Sgt. Marvin H. Morgan is one of two Prince William County police detectives in charge of an investigation that has involved hundreds of hours overtime chasing down leads as obscure as a tip that the murderer fled the murder scene in a helicopter. He and the 22 other police officers who have worked on the case have yet to come up with a motive, a murder weapon or a manor suspect.

When Morgan arrived at the tailer June 24 he found three young women lying face down side by side, each with a single bullet hole in the back of her head. There was no sign of a struggle; the women had not been sexually molested, police said.

"The person who did it had these women lie down and he shot them rather cooly. There was no evidence of the overkill you see when a murderer kills in a rage," Morgan said.

Murder in Prince William County, a rapidly developing Virginia jurisdiction on the southern edge of the Wastington suburbs, isn't uncommon. the county of 128,500 people typically averages o/ne murder a month, said Commonwealth's Attorney Paul B. Ebert.

Yet, most murders in the past, according to Morgan, have been simple crimes of rage or robbery. "In most of the homicides we have around here," Morgan said, "You can establish a motive immediately at the murder scene.

"But at this murder (in Dale City) we were shocked, no, I guess we were bewildered because there didn't seem to be a motive."

The Federal Bureau of Investigation, called in to assist the investigation, has studied the murders and concluded they were committed by a professional killer, according to prosecutor Elbert. The few suspects who have come up in the investigation are hardly professional killers, he said.

Elbert, who talks by phone with police three to four time a day about the investigation, said police have toyed with a number of theories about how the murders happened.

Two of the victims, Sharon Lake, a 25-year-old school teacher from Dale City, and a friend, 23-yar-old Deborah Warner Frank from Alexandria, were house hunting when they went to the Ryland Homes sales trailer near the dead end of Dale Bouvard, police have said.

Working in the trailer was 17-years-old Karen Rose Scarbrough, a Falmouth, Va., girl on her first day of her first job after graduating from high school.

Police have theorized that the murderer may have been starting to sexually attack Scarbrough when he was surprised by the two other women entering the trailer, Ebert said. Or perhaps the murderer had been following the two women around Dale City that afternoon as they shopped for a house, police have said.

Police are checking to see if there are disgruntled employes or clients of Ryland Homes, a major Washington builder who may have wanted to kill another women who normally worked on weekends at the trailer. They've eliminated a pershon who was seen at the trailer a week before the murder and who "acted strangely" toward the Ryland employes, Morgan said.

"We have a few good suspects and with the right interview we may be able to make an arrest in a couple weks," Ebert said. But he added that the investigation is likely to go on for several months as investigations follow up on leads called in by citizens.

Thus far the investigation has run into dead ends on several well publizea clues. A card filled out in the trailer on the day of the murder by a prospective home buyer and found by police has producted nothing. Morgan said. The name, address and phone numer on the card are fake, Morgan said, and efforts to trace the hand writing have been fruitless.

A tip about a red Volkswagen seen in the area at time of the murder has gone nowhere. Morgan said.