Loudoun County police arrested a Woodbridge man yesterday in the Wednesday night stabbing death of a Sterling teenager, the county's first homicide in five years, police said.

Jason Schoffstall, 18, a graduate of Park View High School, died of a chest wound on the way to Washington Hospital Center, police said. Friends say he was chasing a truck on foot when a man inside lunged through a window and stabbed him with a knife.

Stephen J. Roman, 24, was arrested at 5:35 a.m. yesterday at his home and charged with second-degree murder, police said. Roman was later released on $5,000 bail, police said.

Officials said Schoffstall's slaying is the first in the county since Charles Curtis murdered Judith L. DeMaria near a Loudoun County bicycle path in August 1985. Curtis, a drifter, later pleaded guilty.

The most publicized case in recent memory, police said, was the murder trial of former American University official William "Bull" Evans-Smith, who was convicted of killing his wife of 43 years at their Loudoun County estate in April 1985.

Loudoun, which is about 25 miles west of the District, still is a largely rural county that has had fast-paced development in the last decade. Police and a county prosecutor said they did not understand the five-year lapse in homicide cases. They said five years is a long time without any major crime, considering the growth in areas such as Sterling on the county's eastern end.

Sgt. Larry Wilkins, a former Fairfax officer who now is a Loudoun sheriff's deputy, said he expects that Loudoun will begin to experience more slayings just as neighboring Fairfax County did in recent years. Residents in the area, however, are uneasy.

"Everybody is real frantic about it," said Jason Dodge. "He's {Schoffstall} been here forever."

"I think a lot of people are pretty upset by it," said Steve Schnell, 30, who, at the time of the stabbing, was working in the Hop-In Convenience store, which is in an area of apartments and single family homes on East Holley Avenue in Sterling. "It's an absolute total waste of life," Schnell said.

Schoffstall's slaying occurred in the parking lot of the convenience store. Other youths at the scene said they and Schoffstall twice traded insults with two men in a four-wheel-drive truck, but did not mean to fight them.

Rick Pearson, who lives off East Holly Avenue and had just bought a package of incense at the store, said he saw Schoffstall and his friends run after the truck when the men drove away. Pearson, 34, said most of the group's members stopped chasing the truck and walked back to their cars. Schoffstall continued. He said the teenager then walked back toward the store, looking at his blood-soaked shirt. "He goes, 'Look what they did to me' . . . . Blood was just coming out . . . . The next thing he says, 'Everything is getting black.' "