Loudoun's top prosecutor said yesterday he has concluded that the fatal shooting in March of a 15-year-old Ashburn boy by his 17-year-old friend and former classmate was accidental.

Matthew J. Lathram is expected to plead guilty to involuntary manslaughter in the March 22 slaying of Donald Nicholas Shomaker, Commonwealth's Attorney James E. Plowman said. Lathram has also agreed to plead guilty to several drug and weapons charges, he said.

Plowman's comments came yesterday after Lathram waived his right to a preliminary hearing in juvenile court and a judge certified the case to adult court. Plowman said he will not pursue a murder charge and indict Lathram on involuntary manslaughter and other charges in July.

Lathram and Shomaker, close friends who often spent time together, were in the basement of Lathram's Ashburn home on a Monday evening when Lathram fired a single shot from a Glock semiautomatic that was under his shirt. The shot pierced Shomaker's chest, according to court documents and sheriff's officials. Shomaker, a freshman at Broad Run High School, died that evening.

Lathram's attorney, Alex Levay, said that his client has been distraught over the shooting and confirmed that Lathram intends to plead guilty of involuntary manslaughter, a crime that carries a maximum 10-year sentence.

"Everyone should realize that my client has been devastated by the loss of his dear friend. He takes some solace in the determination that this tragic death was an accident," Levay said in a statement. "We all pray for Nick and his family."

Plowman declined to say what prompted the shooting or comment on evidence that led him to seek a lesser charge. But he said information gathered by investigators has convinced him that the shooting was not intentional.

"An investigation has been done very thoroughly, and now we're comfortable that this was simply an accident," Plowman said. He stressed that Lathram will also be prosecuted on drug and weapons charges. He called the shooting "one of the inherent results when you dabble in that activity."

Sheriff's officials said they had been called to the house that Lathram shared with his grandparents several times during the past several years for reports of gunshots and suspicions that drugs were being sold. Detectives who searched the house after the slaying found cocaine, marijuana and hallucinogenic mushrooms as well as three guns and about $1,700.

The shooting shocked neighbors in the quiet Ashburn community, where serious crime is rare. The school held an assembly on violence prevention, and more than 200 people gathered for a march to honor Shomaker, who was known for his infectious grin but had a difficult time transitioning to high school.

Friends and authorities said Shomaker had become close with Lathram, who had attended Broad Run High School but was enrolled in an alternative program at the time of the shootings.

Lathram's neighbors said they often saw young men hanging out at the house and were worried that illegal activity was taking place. Anna Chamberlain, 75, who lives directly across the street, said the teenagers were there so often that at times she was afraid to leave her house.

When Lathram first moved in with his grandparents, Chamberlain said, he was a skinny, shy boy whom she sometimes watched after school. "He was very quiet. He just wanted milk, cookies and TV," she said. As the boy entered high school, though, he changed, she said.

Chamberlain said she was watching out her front window the night of the shooting. About a half-hour before it happened, Chamberlain said, she saw three boys hanging out in front of the house. "It was cold, and it was windy," she recalled. "They were sort of huddled on the front porch. Every so often, they'd come out onto the yard and box a little -- I just thought to keep warm."

Sheriff's deputies and an ambulance arrived at the house a little later, and she realized something terrible had happened when she saw Lathram guide rescue workers toward a side entrance to a basement. Later, Chamberlain said, she saw him sitting, head in his hands, saying that the shooting had been an accident.