Murders in Prince William County increased by nearly two-thirds last year, giving it the highest homicide rate in Northern Virginia in 1982, statistics show.

Eleven people in Prince William were murdered in 1982, up from seven the year before and representing an eleven-fold increase over 1980 and 1981, county police said. The remains of a 12th possible homicide victim were also discovered last year, police said, but it was unclear whether the death had occurred in 1982 or in previous years.

Meanwhile, other crimes in Prince William, such as car thefts, burglaries and assaults, remained steady or dropped in the same year, they said.

"To be perfectly honest, I don't know why we had so many murders," said Police Chief George Owens, who has run the 239-member department since it was created in 1970. "I don't have any reason to believe this year will be as bad. But this is the type of crime where our presence does not really have an effect."

Three of the homicides are still under investigation, police said, but in the remaining cases the victims either knew, or were related to, the persons accused of killing them.

Two persons have already been convicted in county court and sentenced to die. John LaVasseur, 19, was found guilty of beating and stabbing to death Pamela Benner, 19, in her Woodbridge home last February. And in an unrelated case, Timothy D. Bunch, 21, a marine on temporary duty at the Quantico base, was convicted in the February shooting death of Su Cha Thomas, 40. Both men are appealing their convictions.

Those death sentences were the first to be handed down in Prince William in 61 years. Only four people were sentenced to death in Virginia last year, police said.

Neighboring counties report that their homicide rate in 1982 remained stable: nine were recorded in Fairfax County, which has a population of 620,000; six in Alexandria, with a population of 110,000; and six in Arlington County, which has 160,000 residents. Slightly more than 140,000 people live in Prince William, a part-rural, part-suburban bedroom community about 30 miles due west of Washington.

"This wasn't the bogyman coming down the street late at night," said Prince William Police Captain James K. Sullivan, who heads the county's investigation department, of the increase in homicides. "I can comprehend why each of these happened: the despair of a father, the passion of a husband.

"This wasn't a crime spree. Just coincidences of human nature."