Not long after she got home from school Monday, 13-year-old Ann Marie Harrison of Woodbridge was playing with a few neighborhood friends on the sidewalk, throwing snowballs and shivering in the late winter dusk.

Her parents weren't home from work and she was defying one of their rules by playing outside. "She kept saying: 'I'm gonna get in trouble, I'm not supposed to be outside,' " said her friend and eighth grade classmate Shelley Wilkerson.

That's the last Shelley saw of her friend -- until an ambulance arrived a half-hour later and the paramedics carried Ann out of a town house in Rollingwood Village on a stretcher. The victim of an accidental shooting, Ann died several minutes later in Potomac Hospital from a single shot in the chest.

"She was so nice, unbelievably nice," Shelley said yesterday, breaking into tears. "She didn't do anything to anybody ever. She didn't deserve this. Nobody can believe she's dead, she's gone, she's not coming back."

The shooting about 5 p.m. Monday was the result of what police regard as a potentially deadly mix: juveniles and a loaded weapon. It jolted the Prince William County neighborhood and stripped away a veneer of security that many residents said they believed was assured by the fact that an FBI agent and a county policeman were among their neighbors.

According to police, Ann was at the home of a 17-year-old neighbor, who was showing her and a 15-year-old boy his father's handgun, a .44-caliber magnum. None of the youths evidently realized the gun was loaded, and no adults were present.

The 17-year-old passed the gun to the younger boy, who pulled the trigger, hitting Ann in the chest.

Although neighbors said the boys first told them that an unknown male had entered the house, fired the shot and run off, they later told police how the shooting occurred. No charges have been brought pending an autopsy, police said, and the shooting is regarded as accidental.

Yesterday at Ann's home, a two-story gray town house with black trim, her parents sat at the kitchen table under a tapestry likeness of Leonardo Da Vinci's Last Supper, consumed with memories of their only daughter.

They recalled the verve she injected into the household, her constant telephone chatter, her ever-shifting wardrobe of borrowed clothing, the tiffs over school, where she was a poor student, and the discordant sounds of her clarinet practice.

"She couldn't keep quiet a minute," said Tony Harrison, her adoptive father. "She thought that school was the biggest social club ever invented."

"It was a senseless . . . act. It's taken Ann from us and it's destroyed those two boys," said Harrison. "They shouldn't expect kids to do the right thing." He said he would have to guard against the tendency to be overprotective with his 3-year-old son Steven.

In the neighborhood, a number of residents said they were struggling to explain to their own children what had happened.

"I don't like any weapons whatever," said Joan E. Mason. "Either little kids or big kids -- if they get a hold of a gun and it goes off you just don't know what's going to happen.

"It's a tragedy, it really is," she said. "It's going to be sad around here for a long time."